Sometimes I get a Bad Request 400 error at the GMail login screen (or, if I'm already logged in, the GMail UI can't do anything requiring server interaction). Clearing cookies solves it temporarily.
Surely this is a duplicate, but scanning a few search result lists didn't turn anything up. Switched to Firefox once because of this, trying the Safari 4 beta but the problem is still there and will probably give in again. Here are lots of reports:
Cookies are handled by the system CFNetwork framework, not by WebKit. You should file bug reports against CFNetwork at <http://bugreport.apple.com/>.
Thanks, reported via ADC.