Here's how you reproduce this bug. Take an album which has two entries in MB with an identical name but different number of tracks (i.e. Bjork Homogenic is listed twice. One with 10 tracks and one with 16 tracks). Now drop all the MP3 files of the album onto MB for identification (use the one with the smallest # of tracks. in this case I am using the album with 10 tracks). If one is mis-identified and you accidentally tag it with a track from the album with 16 tracks (and you only have 10), then when you hit save, ALL MP3 files will be corrupted and are no longer usable.
