That initial login / signup form doesn't check for duplicate usernames. I can enter a username that is already taken and proceed to the registration form. It doesn't give me an error about the username until I submit the registration form.
Also, the tooltip for duplicate usernames on the registration form doesn't activate because the field is already filled in and there is no action to trigger it.
The design of the inline validation makes this hard to workaround in the "ideal" way, but it does now check your name/email validity on the initial page load and show an error message if they're invalid/taken.