Don't cancel the FlyAttack activity if the activity has already been canceled. The Tick routine calls Cancel() on FlyAttack, which removes any NextActivity that may have been set by an outside listener. This is the case with a rare bug causing planes to be killed, removed from the playable game, but still existing in the game world.

OpenRA.Mods.RA/Air/FlyAttack.cs

@@ -46,10 +46,13 @@ namespace OpenRA.Mods.RA.Air
 
 		public override void Cancel( Actor self )
 		{
-			if( inner != null )
-				inner.Cancel( self );
+			if( !IsCanceled )
+			{
+				if( inner != null )
+					inner.Cancel( self );
 
-			base.Cancel( self );
+				base.Cancel( self );
+			}
 		}
 	}
 }