"iTunes Sync: X items could not be synced. See iTunes for more information."
I haven't got a solution to this yet, but I get the same error (342 items though). The reason appears to be that the iPhone contains 342 hidden voice memos, not appearing within the voice memo app, but viewable (and playable while the iPhone is connected through USB) in the iPhone Device list in iTunes (click the triangle left to the device, see the voice memo playlist).

I will backup and restore to factory setting now. But I'm fearing that the voice memos will be back there when I restore the backup. But then again, I might be lucky.. :) 

UPDATE; It did in fact work, I did get rid of the hidden voice memos and the "could not sync" message! Great.

Thoughts on how the hidden memos got there in the first place; I did mess around with the location of the Voice Memos playlist in iTunes, moved it in under a playlist folder, back and forth a couple of times, synced between, in addition to choosing to sync voice memos and not. It was all me testing how the (not intuitive) Voice Memos syncing works. Not sure if this produced the problems though.. but date stamps etc support the thoughts..