I had the same problem with a .rm(realplayer) file, and decided to take it to my Wintel machine and crack the sucka open with HexWorkshop. I noticed that there was some HTML code and "Gnutella Client" stuff.
I opened a similar(rm) file that worked, and simply copied and pasted into the mangled file. Worked perfectly! I haven't tried this with MPEG but I think this is the problem.
I don't know why Mactella does this, maybe its with long files or resumes that it makes this error as I haven't seen this phenom with MP3s.
So since Mac users are generally dumber(less technical?) than average, I recomend trying to download your media again.
Hope this helps!