It's possible for a user to register a user name that displays identically to an existing one, as long as the existing user name contains more than one word. This can lead to impersonation problems.

For example lets state that there's a user name called Joe Bloggs. Another user then registers another account with the same name, but puts two spaces between Joe and Bloggs instead of one. The second name is deemed unique by the registration process, but when displayed only one space will be shown between the two words of the name in most browsers.