I disabled the username field and removed it from showing on the registration page because I was the user to only use their email address.
The problem is that in some circumstances the username field is still being submitted when a new user registers and the customer gets a pop-up that says "User name in use". I haven't figured out if this is only certain browsers or what, but it should never be happening since I disabled it.
The site is
www.abca.org
and I am testing with IE 8 right now. But I have also noticed the issue a few times in the latest version of Firefox.