incompletes you say? The possibilities I can think of are
1. Fakes. Beware of Fake files in search results
2. Deliberately corrupted files. mp3 Bit-Rates Versus File Sizes (click on link) (ignore the T3 reference)
3. Files that are being shared from another gnutella client program that don't transfer their data/id3 tags properly.
4. mp3pro files that iTunes is unable to play.
5. A higher than normal (above 320 bitrate) that most players cannot read/play.