Last week I noticed that videos weren't watchable on my iPhone or iPad. I cleared them off and reloaded, but they still didn't transfer successfully. I have seen similar problems before, most recently when I created a new (empty) album in iPhoto and told iTunes to sync it over, without any photos in the album. iTunes would try to sync the empty album and fail, but never identified the album or photo which was causing trouble. I was also getting a lot of (partial) iTunes crashes, which I do not recall from previous problems.
In this case I turned off synching of Movies, TV Shows, Music, Ringtones, and Podcasts. I synched both devices (via USB, which is faster but probably wasn't strictly necessary), clearing off all the problematic content. Then I re-enabled individual types and synched again, hoping to isolate the data type causing trouble. I never found it -- all the content is back on both devices, and video is watchable.
iTunes FAIL.