--- a/lang/english.txt	Sat Sep 16 13:44:12 2006 +0000
+++ b/lang/english.txt	Sat Sep 16 16:21:47 2006 +0000
@@ -2906,6 +2906,8 @@
 STR_REPLACE_ENGINE_WAGON_SELECT                                 :{BLACK}Replacing: {ORANGE}{SKIP}{SKIP}{STRING}
 STR_REPLACE_ENGINE_WAGON_SELECT_HELP                            :{BLACK} EXPERIMENTAL FEATURE {}Switch between engine and wagon replacement windows.{}Wagon replacement will only be done if the new wagon can be refitted into carrying the same type of cargo as the old one. This is checked for each wagon when the actual replacement takes place.
 STR_ENGINE_NOT_BUILDABLE                                        :{WHITE}Engine is not buildable
+STR_AUTOREPLACE_FAILED                                          :{WHITE}BUG: Autoreplace refit failure
+STR_AUTOREPLACE_REFIT_FAILURE                                   :{WHITE}{STRING} replaced into {STRING} failed to refit to {STRING}
 
 STR_ENGINES                                                     :Engines
 STR_WAGONS                                                      :Wagons

--- a/vehicle.c	Sat Sep 16 13:44:12 2006 +0000
+++ b/vehicle.c	Sat Sep 16 16:21:47 2006 +0000
@@ -1778,9 +1778,18 @@
 
 		/* refit if needed */
 		if (replacement_cargo_type != CT_NO_REFIT) {
-			int32 temp_cost = DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type));
-			assert(!CmdFailed(temp_cost)); // assert failure here indicates a bug in GetNewCargoTypeForReplace()
+			if (CmdFailed(DoCommand(0, new_v->index, replacement_cargo_type, DC_EXEC, CMD_REFIT_VEH(new_v->type)))) {
+				/* We should not be here
+				 * being here shows a failure indicates a bug in GetNewCargoTypeForReplace() or incorrect estimation costs */
+				SetDParam(0, GetCustomEngineName(old_v->engine_type));
+				SetDParam(1, GetCustomEngineName(new_v->engine_type));
+				SetDParam(2, _cargoc.names_s[replacement_cargo_type]);
+				ShowInfo("Error: Autoreplace failed to refit (bug)");
+				ShowErrorMessage(STR_AUTOREPLACE_REFIT_FAILURE, STR_AUTOREPLACE_FAILED, 0 ,0);
+				NOT_REACHED(); // stop debug builds at this error for easier debugging
+			}
 		}
+
 		if (new_v->type == VEH_Train && HASBIT(old_v->u.rail.flags, VRF_REVERSE_DIRECTION) && !IsMultiheaded(new_v) && !(new_v->next != NULL && IsArticulatedPart(new_v->next))) {
 			// we are autorenewing to a single engine, so we will turn it as the old one was turned as well
 			SETBIT(new_v->u.rail.flags, VRF_REVERSE_DIRECTION);