I think this could be considered a bug, but I'm err on the side of caution to avoid being flamed just after joinnig /. ;)
When you create an account for the first time, and pick either an invalid Nickname (leading -- dash dash), or one already taken, you are presented with a error message which is not very visible (didn't notice it at first), but more important the page looses the email address entered twice, the toggle selections, and the captcha.
It would be *much* nicer to keep all values except the invalid one, to allow resubmitting without re-entering the valid fields.
PS: FireFox 3.0.4 on winXP32