I think I've had it happen when it wasn't also open in ResEdit. However, I'm not certain.
I know its never happened while using the graphical syst editor; only the more normal syst editors.
A suggestion for the next version: If a resource can't be saved, find that out before closing it, not after. Less risk of losing something you just spent a few minutes on.
(This message has been edited by Lindley (edited 02-04-2004).)