This is a bit involved.
I've been updating album cover art, and found this bug.
If I update anything in the extended tag window, and click >> before the save completes, an exception occurs, and the save dialog will not go away.
Can continue working, so it's not too big a deal, but the ideal solution is to disable the controls in the extended tag window until the save is complete.
The log doesn't show much, but this might help.