For about a month, I've had a problem with my Contacts app crashing on my T-Mobile G1. Whenever this would happen, I would be forced to close the Contacts application. This was a pretty major pain in the butt since I tend to use the Contacts and Favorites tab to do most of my dialing.
It took me a while to figure out that a specific contact was causing the problem. Every time I clicked on this contact, the app would crash. I have no idea what was wrong with the contact, but I deleted and re-added the contact via Gmail's contact management interface. This fixed the problem.