I ran into a new debugging problem where a view controller was not showing anything from it's nib file. It was just transparent. I attempted to debug it, reconnected all the interface builder items. I finally figured out that I had implemented loadView instead of viewDidLoad to do some initialization. This caused the nib file to be ignored, the code was expecting me to programmatically create my view, which I wasn't doing.
There was no obvious error for this one, figuring it out cost a couple of hours.