I get it sometimes by disconnecting my iPhone too quick. If you run the sync and let it sit there, it took all night but finally completed and stopped on its own. Hopefully it was a one time thing to do something and will sync quick now. I will know in a few hours.
I also found that if you get a call and it interrupts a sync, simply going into iTunes and clicking sync again did not finish putting my music on it. I had to remove my music and playlists I used to sync and clear the music and add the sync again and it worked.