Skip registeration (proceed) button tries to validate login if any values has been given to username and password fields.

If user enters values to login pane and presses submit button in continue pane, will login credentials be sent to same validator as it would if proceed button would be pressed in login pane.

Patch i will post on comment below is working, but requires form altering to differentiate the labels of both buttons.

Think better solution would be to allow own validation for both login and continue pane's buttons.

Currently even if both have own buttons and validate methods, gets first two from sample below ignored and ['buttons']['continue']['#validate'] value get's used on both of them.

$form['login_pane']['buttons']['continue']['#validate'];
$form['continue_pane']['buttons']['continue']['#validate'];
$form['buttons']['continue']['#validate'];