I just noticed this on my own board and tested it on xenforo.com. When errors are encountered during the registration process, the registration form does not reload and rather the user is directed to a new page without the registration form. Maybe this is as designed? But, it seems out of the ordinary, so just in case...
To reproduce: (XF 1.1.4)
Use an illegal username (like admin)
Do not fill out any other field, except for the verification question and terms agreement.