In reply to:
I think that if record companies used tactics like that - or inserted little commercials in their mp3s, I would consider that fair play.
One of my favorite bands the Barenaked Ladies did this. With their last album, the first single was released first on napster a good couple of months before it hit the radio stations. The problem was that every thirty seccond they would come on and tell you to buy their CD when it came out. But, this was great fun because the guys are canadian and have the best (bar none) banter of any band, so their interuptions were entertaining... It was completely possible for fans to edit out their sketches, but most did it for themselves and didn't share the edited version. I think I've still got the orginal sitting around somewhere, it was great and showed a great use of Napster by a big label band.
I doubt the file you're talking about had this happen to it accidentlaly... Though many P2P programs merge file fragments, even the most basic ones check CRC/MD5's to make sure the orginal file and the new file are the same. (ok, "same". but very close to same)
Matthew