Just a record for myself and as a reference for other people with this problem. I encountered Error (-50) when syncing my iPhone 5 with iTunes.
(Macbook Air 13 inch, photos from iPhoto and syncing iPhone 5)
The problem only occurred when I tried to try to sync photos to my iPhone 5 through iTunes with photos from iPhoto. I had tried lots of other way of fixing through the internet and it all doesn’t work for me.
And the way that I found at last is from this site.
It suggested that you open up your folder for iPhoto Library-> right click it and select “Show package contents”.
Inside it, you will find a file/folder named “iPod Photo Cache”.
Delete that folder and let iTunes re-sync again for your device.
This way worked for me and I thought I should record it down for future reference. If it doesn’t work for you, sorry and don’t stop trying other methods as suggested from the internet.