Comments accounts were supposed to be case insensitive, but there was one part of the code (mostly affecting reseting passwords) which wasn't; consequently people who entered their accounts with mixed case weren't able to reset their passwords. This has now been fixed.
Also, for some reason I was never able to figure out, once in a great while someone would have problems with cookies and would get told, when trying to post with their comment account, that the display name was already in use. I still don't get what was going on, but I've just disabled the relevant section when a person is using a comment account (because the display name is forced to be what's specified for the comment account), and this seems to have fixed things.