BUGFIX: with PlaceBuilding active, you could cancel the current production, then place the (canceled) building.

OpenRa.Game/PlaceBuilding.cs

@@ -40,6 +40,11 @@ namespace OpenRa.Game
 			}
 		}
 
-		public void Tick() { }
+		public void Tick()
+		{
+			var producing = Owner.Producing( "Building" );
+			if( producing == null || producing.Item != Name || producing.RemainingTime != 0 )
+				Game.world.AddFrameEndTask( _ => { Game.controller.orderGenerator = null; } );
+		}
 	}
 }

OpenRa.Game/UnitOrders.cs

@@ -69,13 +69,14 @@ namespace OpenRa.Game
 					Game.world.AddFrameEndTask( _ =>
 					{
 						var building = Rules.UnitInfo[ order.TargetString ];
+						var producing = order.Player.Producing( "Building" );
+						if( producing == null || producing.Item != order.TargetString || producing.RemainingTime != 0 )
+							return;
+
 						Log.Write( "Player \"{0}\" builds {1}", order.Player.PlayerName, building.Name );
 
-						//Adjust placement for cursor to be in middle
 						Game.world.Add( new Actor( building.Name, order.TargetLocation - GameRules.Footprint.AdjustForBuildingSize( building.Name ), order.Player ) );
 
-						Game.controller.orderGenerator = null;
-
 						order.Player.FinishProduction(Rules.UnitCategory[building.Name]);
 					} );
 					break;