I filed this a few weeks ago and the ticket wasn't commented on nor discussed here, so I replicate it here to bring some attention to it. It's an obscure bug:
============== Calls to account/update_profile API fails if the original screen name (the one that was signed up with, eg. "User1", instead of "user1") has uppercase characters and login don't match exactly (e.g "user1" and not "User1"). Most, if not all other Twitter API calls, including the authenticating call is case-insensitive. But if the user has a screen name which has at least an uppercase character, account/update_profile will fail unless the authenticated screen name is spelled exactly the same. http://code.google.com/p/twitter-api/issues/detail?id=926
