I've found the item that cannot be synced. It's an expired rental movie that still appears on my iPhone. However, every time I delete the movie, and do a sync, it comes back. I've tried differernt things but the deleted movie always comes back.
Here is the deleted movie.

Note that the movie expired months ago.
If I delete the movie, and click apply, the movie goes away. But if I click Sync, it comes back.
If I select set up iTunes to sync movies, or to not sync movies, this doesn't help. If I set up iTunes to manually manage music and videos on the device, this doesn't help either. No matter what I do, the movie always comes back.