Some users who have upgraded to iTunes 8.0.1 on Mac have been haunted by the following message while attempting to sync their iPhone or iTouch:
The [device] cannot be synced. An unknown error occurred (13014).
To resolve the issue, simply close out of iTunes, then open it again. If the error persists, restart your computer completely.
If your iPhone or iTouch is not syncing to iTunes and you’ve just recently upgraded to Leopard, try this fix.
Source: Apple Support