Quite a few, but: as I understand it, our chat software is some kind of add-on to the rest of the system and it sometimes stumbles over usernames that contain punctuation. When a member has such a username, they may find themselves unable to chat except with the the "No Popup Chat".

I used to have a username with a space in it; that didn't cause any problems. Parentheses reportedly have caused problems but I'm not sure what else may have. Pretty much all I have to go by is what I've seen mentioned in this forum so far.