iTunes will stop syncing as if it finished without having done anything.
So when I hooked up my iPod Touch to my Windows PC to sync my new music, it would start the sync as it normally does. But then iTunes will stop responding, and after about 10 minutes of waiting, it will respond again as if the sync was finished, but nothing is added to my iPod.
iPod touch, Windows Vista, iPod Touch 3rd Gen 32 GB