This is an old bug with many reports For example: see http://code.google.com/p/pharo/issues/detail?id=830#c2
Check if StandardFileStream is still in use in your image, specifically in the condenseChanges phase. If yes replace it with FileStream (or MultiByteFileStream, I don't remember), retry a condenseChanges (I hope you saved a working copy of the image/changes pair) and see if it fixes the problem.
Normally, this should have been fixed in recent versions of Pharo, but since there are so many reports of the same bug, I did not took time to dig into the new bugtracker, nor into current development branch.