diff --git a/src/build_vehicle_gui.cpp b/src/build_vehicle_gui.cpp
index e68dc8ccb9..91e32d4258 100644
--- a/src/build_vehicle_gui.cpp
+++ b/src/build_vehicle_gui.cpp
@@ -753,7 +753,11 @@ static int DrawAircraftPurchaseInfo(int left, int right, int y, EngineID engine_
 static uint ShowAdditionalText(int left, int right, int y, EngineID engine)
 {
 	uint16 callback = GetVehicleCallback(CBID_VEHICLE_ADDITIONAL_TEXT, 0, 0, engine, NULL);
-	if (callback == CALLBACK_FAILED) return y;
+	if (callback == CALLBACK_FAILED || callback == 0x400) return y;
+	if (callback > 0x400) {
+		ErrorUnknownCallbackResult(Engine::Get(engine)->GetGRFID(), CBID_VEHICLE_ADDITIONAL_TEXT, callback);
+		return y;
+	}
 
 	StartTextRefStackUsage(6);
 	uint result = DrawStringMultiLine(left, right, y, INT32_MAX, GetGRFStringID(Engine::Get(engine)->GetGRFID(), 0xD000 + callback), TC_BLACK);
diff --git a/src/industry_gui.cpp b/src/industry_gui.cpp
index d991c5c70e..1ea3967fd4 100644
--- a/src/industry_gui.cpp
+++ b/src/industry_gui.cpp
@@ -73,7 +73,10 @@ static void GetCargoSuffix(uint cargo, CargoSuffixType cst, const Industry *ind,
 	suffix[0] = '\0';
 	if (HasBit(indspec->callback_mask, CBM_IND_CARGO_SUFFIX)) {
 		uint16 callback = GetIndustryCallback(CBID_INDUSTRY_CARGO_SUFFIX, 0, (cst << 8) | cargo, const_cast<Industry *>(ind), ind_type, (cst != CST_FUND) ? ind->location.tile : INVALID_TILE);
-		if (GB(callback, 0, 8) != 0xFF) {
+		if (callback == CALLBACK_FAILED || callback == 0x400) return;
+		if (callback > 0x400) {
+			ErrorUnknownCallbackResult(indspec->grf_prop.grffile->grfid, CBID_INDUSTRY_CARGO_SUFFIX, callback);
+		} else if (indspec->grf_prop.grffile->grf_version >= 8 || GB(callback, 0, 8) != 0xFF) {
 			StartTextRefStackUsage(6);
 			GetString(suffix, GetGRFStringID(indspec->grf_prop.grffile->grfid, 0xD000 + callback), suffix_last);
 			StopTextRefStackUsage();
@@ -455,12 +458,16 @@ public:
 				str = STR_NULL;
 				if (HasBit(indsp->callback_mask, CBM_IND_FUND_MORE_TEXT)) {
 					uint16 callback_res = GetIndustryCallback(CBID_INDUSTRY_FUND_MORE_TEXT, 0, 0, NULL, this->selected_type, INVALID_TILE);
-					if (callback_res != CALLBACK_FAILED) {  // Did it fail?
-						str = GetGRFStringID(indsp->grf_prop.grffile->grfid, 0xD000 + callback_res);  // No. here's the new string
-						if (str != STR_UNDEFINED) {
-							StartTextRefStackUsage(6);
-							DrawStringMultiLine(left, right, y, bottom, str, TC_YELLOW);
-							StopTextRefStackUsage();
+					if (callback_res != CALLBACK_FAILED && callback_res != 0x400) {
+						if (callback_res > 0x400) {
+							ErrorUnknownCallbackResult(indsp->grf_prop.grffile->grfid, CBID_INDUSTRY_FUND_MORE_TEXT, callback_res);
+						} else {
+							str = GetGRFStringID(indsp->grf_prop.grffile->grfid, 0xD000 + callback_res);  // No. here's the new string
+							if (str != STR_UNDEFINED) {
+								StartTextRefStackUsage(6);
+								DrawStringMultiLine(left, right, y, bottom, str, TC_YELLOW);
+								StopTextRefStackUsage();
+							}
 						}
 					}
 				}
@@ -783,17 +790,21 @@ public:
 		/* Get the extra message for the GUI */
 		if (HasBit(ind->callback_mask, CBM_IND_WINDOW_MORE_TEXT)) {
 			uint16 callback_res = GetIndustryCallback(CBID_INDUSTRY_WINDOW_MORE_TEXT, 0, 0, i, i->type, i->location.tile);
-			if (callback_res != CALLBACK_FAILED) {
-				StringID message = GetGRFStringID(ind->grf_prop.grffile->grfid, 0xD000 + callback_res);
-				if (message != STR_NULL && message != STR_UNDEFINED) {
-					y += WD_PAR_VSEP_WIDE;
+			if (callback_res != CALLBACK_FAILED && callback_res != 0x400) {
+				if (callback_res > 0x400) {
+					ErrorUnknownCallbackResult(ind->grf_prop.grffile->grfid, CBID_INDUSTRY_WINDOW_MORE_TEXT, callback_res);
+				} else {
+					StringID message = GetGRFStringID(ind->grf_prop.grffile->grfid, 0xD000 + callback_res);
+					if (message != STR_NULL && message != STR_UNDEFINED) {
+						y += WD_PAR_VSEP_WIDE;
 
-					StartTextRefStackUsage(6);
-					/* Use all the available space left from where we stand up to the
-					 * end of the window. We ALSO enlarge the window if needed, so we
-					 * can 'go' wild with the bottom of the window. */
-					y = DrawStringMultiLine(left + WD_FRAMERECT_LEFT, right - WD_FRAMERECT_RIGHT, y, UINT16_MAX, message, TC_BLACK);
-					StopTextRefStackUsage();
+						StartTextRefStackUsage(6);
+						/* Use all the available space left from where we stand up to the
+						 * end of the window. We ALSO enlarge the window if needed, so we
+						 * can 'go' wild with the bottom of the window. */
+						y = DrawStringMultiLine(left + WD_FRAMERECT_LEFT, right - WD_FRAMERECT_RIGHT, y, UINT16_MAX, message, TC_BLACK);
+						StopTextRefStackUsage();
+					}
 				}
 			}
 		}
diff --git a/src/newgrf_airport.cpp b/src/newgrf_airport.cpp
index 05e54ab230..9137bf8657 100644
--- a/src/newgrf_airport.cpp
+++ b/src/newgrf_airport.cpp
@@ -273,7 +273,12 @@ StringID GetAirportTextCallback(const AirportSpec *as, byte layout, uint16 callb
 	object.callback = (CallbackID)callback;
 
 	group = SpriteGroup::Resolve(as->grf_prop.spritegroup[0], &object);
-	if (group == NULL) return STR_UNDEFINED;
+	uint16 cb_res = (group != NULL) ? group->GetCallbackResult() : CALLBACK_FAILED;
+	if (cb_res == CALLBACK_FAILED || cb_res == 0x400) return STR_UNDEFINED;
+	if (cb_res > 0x400) {
+		ErrorUnknownCallbackResult(as->grf_prop.grffile->grfid, callback, cb_res);
+		return STR_UNDEFINED;
+	}
 
-	return GetGRFStringID(as->grf_prop.grffile->grfid, 0xD000 + group->GetCallbackResult());
+	return GetGRFStringID(as->grf_prop.grffile->grfid, 0xD000 + cb_res);
 }
diff --git a/src/newgrf_commons.cpp b/src/newgrf_commons.cpp
index 6d81676dea..2f49c51c76 100644
--- a/src/newgrf_commons.cpp
+++ b/src/newgrf_commons.cpp
@@ -480,17 +480,24 @@ uint32 GetCompanyInfo(CompanyID owner, const Livery *l)
 CommandCost GetErrorMessageFromLocationCallbackResult(uint16 cb_res, uint32 grfid, StringID default_error)
 {
 	CommandCost res;
-	switch (cb_res) {
-		case 0x400: return res; // No error.
-		case 0x401: res = CommandCost(default_error); break;
-		case 0x402: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_IN_RAINFOREST); break;
-		case 0x403: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_IN_DESERT); break;
-		case 0x404: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_ABOVE_SNOW_LINE); break;
-		case 0x405: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_BELOW_SNOW_LINE); break;
-		case 0x406: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_SEA); break;
-		case 0x407: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_CANAL); break;
-		case 0x408: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_RIVER); break;
-		default:    res = CommandCost(GetGRFStringID(grfid, 0xD000 + cb_res)); break;
+
+	if (cb_res < 0x400) {
+		res = CommandCost(GetGRFStringID(grfid, 0xD000 + cb_res));
+	} else {
+		switch (cb_res) {
+			case 0x400: return res; // No error.
+
+			default:    // unknown reason -> default error
+			case 0x401: res = CommandCost(default_error); break;
+
+			case 0x402: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_IN_RAINFOREST); break;
+			case 0x403: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_IN_DESERT); break;
+			case 0x404: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_ABOVE_SNOW_LINE); break;
+			case 0x405: res = CommandCost(STR_ERROR_CAN_ONLY_BE_BUILT_BELOW_SNOW_LINE); break;
+			case 0x406: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_SEA); break;
+			case 0x407: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_CANAL); break;
+			case 0x408: res = CommandCost(STR_ERROR_CAN_T_BUILD_ON_RIVER); break;
+		}
 	}
 
 	/* Copy some parameters from the registers to the error message text ref. stack */
diff --git a/src/object_gui.cpp b/src/object_gui.cpp
index a1bbf33750..6b89c4784f 100644
--- a/src/object_gui.cpp
+++ b/src/object_gui.cpp
@@ -233,19 +233,23 @@ public:
 				/* Get the extra message for the GUI */
 				if (HasBit(spec->callback_mask, CBM_OBJ_FUND_MORE_TEXT)) {
 					uint16 callback_res = GetObjectCallback(CBID_OBJECT_FUND_MORE_TEXT, 0, 0, spec, NULL, INVALID_TILE, _selected_object_view);
-					if (callback_res != CALLBACK_FAILED) {
-						StringID message = GetGRFStringID(spec->grf_prop.grffile->grfid, 0xD000 + callback_res);
-						if (message != STR_NULL && message != STR_UNDEFINED) {
-							StartTextRefStackUsage(6);
-							/* Use all the available space left from where we stand up to the
-							 * end of the window. We ALSO enlarge the window if needed, so we
-							 * can 'go' wild with the bottom of the window. */
-							int y = DrawStringMultiLine(r.left, r.right, r.top, UINT16_MAX, message, TC_ORANGE) - r.top;
-							StopTextRefStackUsage();
-							if (y > this->info_height) {
-								BuildObjectWindow *bow = const_cast<BuildObjectWindow *>(this);
-								bow->info_height = y + 2;
-								bow->ReInit();
+					if (callback_res != CALLBACK_FAILED && callback_res != 0x400) {
+						if (callback_res > 0x400) {
+							ErrorUnknownCallbackResult(spec->grf_prop.grffile->grfid, CBID_OBJECT_FUND_MORE_TEXT, callback_res);
+						} else {
+							StringID message = GetGRFStringID(spec->grf_prop.grffile->grfid, 0xD000 + callback_res);
+							if (message != STR_NULL && message != STR_UNDEFINED) {
+								StartTextRefStackUsage(6);
+								/* Use all the available space left from where we stand up to the
+								 * end of the window. We ALSO enlarge the window if needed, so we
+								 * can 'go' wild with the bottom of the window. */
+								int y = DrawStringMultiLine(r.left, r.right, r.top, UINT16_MAX, message, TC_ORANGE) - r.top;
+								StopTextRefStackUsage();
+								if (y > this->info_height) {
+									BuildObjectWindow *bow = const_cast<BuildObjectWindow *>(this);
+									bow->info_height = y + 2;
+									bow->ReInit();
+								}
 							}
 						}
 					}
diff --git a/src/town_cmd.cpp b/src/town_cmd.cpp
index 9fde9163ac..eb741923b3 100644
--- a/src/town_cmd.cpp
+++ b/src/town_cmd.cpp
@@ -650,10 +650,14 @@ static void GetTileDesc_Town(TileIndex tile, TileDesc *td)
 	td->str = hs->building_name;
 
 	uint16 callback_res = GetHouseCallback(CBID_HOUSE_CUSTOM_NAME, house_completed ? 1 : 0, 0, house, Town::GetByTile(tile), tile);
-	if (callback_res != CALLBACK_FAILED) {
-		StringID new_name = GetGRFStringID(hs->grf_prop.grffile->grfid, 0xD000 + callback_res);
-		if (new_name != STR_NULL && new_name != STR_UNDEFINED) {
-			td->str = new_name;
+	if (callback_res != CALLBACK_FAILED && callback_res != 0x400) {
+		if (callback_res > 0x400) {
+			ErrorUnknownCallbackResult(hs->grf_prop.grffile->grfid, CBID_HOUSE_CUSTOM_NAME, callback_res);
+		} else {
+			StringID new_name = GetGRFStringID(hs->grf_prop.grffile->grfid, 0xD000 + callback_res);
+			if (new_name != STR_NULL && new_name != STR_UNDEFINED) {
+				td->str = new_name;
+			}
 		}
 	}
 
diff --git a/src/train_cmd.cpp b/src/train_cmd.cpp
index 7f06d6adba..3bcfcc64ac 100644
--- a/src/train_cmd.cpp
+++ b/src/train_cmd.cpp
@@ -976,8 +976,26 @@ static CommandCost CheckTrainAttachment(Train *t)
 				/* A failing callback means everything is okay */
 				StringID error = STR_NULL;
 
-				if (callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
-				if (callback  < 0xFD) error = GetGRFStringID(head->GetGRFID(), 0xD000 + callback);
+				if (t->GetGRF()->grf_version < 8) {
+					if (callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
+					if (callback  < 0xFD) error = GetGRFStringID(head->GetGRFID(), 0xD000 + callback);
+					if (callback >= 0x100) ErrorUnknownCallbackResult(head->GetGRFID(), CBID_TRAIN_ALLOW_WAGON_ATTACH, callback);
+				} else {
+					if (callback < 0x400) {
+						error = GetGRFStringID(head->GetGRFID(), 0xD000 + callback);
+					} else {
+						switch (callback) {
+							case 0x400: // allow
+							case 0x401: // allow if railtypes match (always the case for OpenTTD)
+								break;
+
+							default:    // unknown reason -> disallow
+							case 0x402: // disallow attaching
+								error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
+								break;
+						}
+					}
+				}
 
 				if (error != STR_NULL) return_cmd_error(error);
 			}
diff --git a/src/vehicle_cmd.cpp b/src/vehicle_cmd.cpp
index a237b19416..089fb0ec4d 100644
--- a/src/vehicle_cmd.cpp
+++ b/src/vehicle_cmd.cpp
@@ -491,12 +491,30 @@ CommandCost CmdStartStopVehicle(TileIndex tile, DoCommandFlag flags, uint32 p1,
 		default: return CMD_ERROR;
 	}
 
-	/* Check if this vehicle can be started/stopped. The callback will fail or
-	 * return 0xFF if it can. */
-	uint16 callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
-	if (callback != CALLBACK_FAILED && GB(callback, 0, 8) != 0xFF && HasBit(p2, 0)) {
-		StringID error = GetGRFStringID(v->GetGRFID(), 0xD000 + callback);
-		return_cmd_error(error);
+	if (HasBit(p2, 0)) {
+		/* Check if this vehicle can be started/stopped. Failure means 'allow'. */
+		uint16 callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+		StringID error = STR_NULL;
+		if (callback != CALLBACK_FAILED) {
+			if (v->GetGRF()->grf_version < 8) {
+				/* 8 bit result 0xFF means 'allow' */
+				if (callback < 0x400 && GB(callback, 0, 8) != 0xFF) error = GetGRFStringID(v->GetGRFID(), 0xD000 + callback);
+			} else {
+				if (callback < 0x400) {
+					error = GetGRFStringID(v->GetGRFID(), 0xD000 + callback);
+				} else {
+					switch (callback) {
+						case 0x400: // allow
+							break;
+
+						default: // unknown reason -> disallow
+							error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
+							break;
+					}
+				}
+			}
+		}
+		if (error != STR_NULL) return_cmd_error(error);
 	}
 
 	if (flags & DC_EXEC) {
diff --git a/src/vehicle_gui.cpp b/src/vehicle_gui.cpp
index abd1e3317e..cbf6e1c08c 100644
--- a/src/vehicle_gui.cpp
+++ b/src/vehicle_gui.cpp
@@ -253,7 +253,10 @@ byte GetBestFittingSubType(Vehicle *v_from, Vehicle *v_for, CargoID dest_cargo_t
 		v_for->InvalidateNewGRFCache();
 		uint16 callback = GetVehicleCallback(CBID_VEHICLE_CARGO_SUFFIX, 0, 0, v_for->engine_type, v_for);
 
-		if (callback == 0xFF) callback = CALLBACK_FAILED;
+		if (callback != CALLBACK_FAILED) {
+			if (callback > 0x400) ErrorUnknownCallbackResult(v_for->GetGRFID(), CBID_VEHICLE_CARGO_SUFFIX, callback);
+			if (callback >= 0x400 || (v_for->GetGRF()->grf_version < 8 && callback == 0xFF)) callback = CALLBACK_FAILED;
+		}
 		if (callback == CALLBACK_FAILED) break;
 
 		if (GetCargoSubtypeText(v_for) != expected_string) continue;
@@ -332,7 +335,7 @@ static void DrawVehicleRefitWindow(const SubtypeList list[NUM_CARGO], int sel, u
 			/* Get the cargo name. */
 			SetDParam(0, CargoSpec::Get(refit.cargo)->name);
 			/* If the callback succeeded, draw the cargo suffix. */
-			if (refit.value != CALLBACK_FAILED) {
+			if (refit.value != CALLBACK_FAILED && refit.value < 0x400) {
 				SetDParam(1, GetGRFStringID(refit.engine->GetGRFID(), 0xD000 + refit.value));
 				DrawString(r.left + WD_MATRIX_LEFT, r.right - WD_MATRIX_RIGHT, y, STR_JUST_STRING_SPACE_STRING, colour);
 			} else {
@@ -427,7 +430,10 @@ struct RefitWindow : public Window {
 						v->InvalidateNewGRFCache();
 						uint16 callback = GetVehicleCallback(CBID_VEHICLE_CARGO_SUFFIX, 0, 0, v->engine_type, v);
 
-						if (callback == 0xFF) callback = CALLBACK_FAILED;
+						if (callback != CALLBACK_FAILED) {
+							if (callback > 0x400) ErrorUnknownCallbackResult(v->GetGRFID(), CBID_VEHICLE_CARGO_SUFFIX, callback);
+							if (callback >= 0x400 || (v->GetGRF()->grf_version < 8 && callback == 0xFF)) callback = CALLBACK_FAILED;
+						}
 						if (refit_cyc != 0 && callback == CALLBACK_FAILED) break;
 
 						RefitOption option;
@@ -983,6 +989,10 @@ StringID GetCargoSubtypeText(const Vehicle *v)
 {
 	if (HasBit(EngInfo(v->engine_type)->callback_mask, CBM_VEHICLE_CARGO_SUFFIX)) {
 		uint16 cb = GetVehicleCallback(CBID_VEHICLE_CARGO_SUFFIX, 0, 0, v->engine_type, v);
+		if (cb != CALLBACK_FAILED) {
+			if (cb > 0x400) ErrorUnknownCallbackResult(v->GetGRFID(), CBID_VEHICLE_CARGO_SUFFIX, cb);
+			if (cb >= 0x400 || (v->GetGRF()->grf_version < 8 && cb == 0xFF)) cb = CALLBACK_FAILED;
+		}
 		if (cb != CALLBACK_FAILED) {
 			return GetGRFStringID(v->GetGRFID(), 0xD000 + cb);
 		}