>I did occasionally have this happen on a system and the only work around I could get was (unfortunately) restarting the server.

There's a way through the management console, computer management, shares - and there you see all files open by anyone. Select any that you need and just close them (rightclick, disconnect). There are cases when the app is just stuck in memory and won't quit, but it isn't visible and the user doesn't know how to kill it with task manager, or it's after hours and the user is gone.