I sometimes find that when I am adding track numbers manually within the mp3tag grid, and then resort by track number, the track numbers get corrupted. By this, I mean that the track tag for another file gets changed, and I end up with duplicate track numbers.
It's a little hard to explain, and next time it happens I will try to record an exact sequence, but I was wondering if this is a known issue.
I have seen this through many releases.