I saw this: -
An unknown error occurred (-50)
from an iPad Mini whilst trying to sync, via USB, from iTunes 11.3.1 on my Mac.
Thankfully, a few Google searches later, I performed the most minor of resets - General> Reset> Reset All Settings, which appears to have cleared the problem.
I did, it must be said, have an iTunes backup of the little fella, just in case :-)