How to correctly download WWDC videos from iTunes U?
I had the same problem and setting iTunes to run in 32-bit mode fixed it for me. There might be an issue with the video encoding on some of the older WWDC videos like the "Working with Core Data" video that can be found at the Apple developer video site under iOS.

To set iTunes to run in 32-bit mode:


Quit iTunes.
Open your applications folder and control-click or right-click on the iTunes icon.
Select "Get Info".
Click the checkbox next to "Open in 32-bit mode".
Open iTunes again and you should be able to download the videos to your collection.


When you are done with your videos, you can always reverse these steps to set iTunes back to 64-bit mode.