Actually its not really a big slip up. You can see the same thing on the series from time to time. The reason is, Burton can't see with the contacts used to make his eyes white. So for that reason he don't wear them with the visor. The visor its self is made so he can see through it, the reason you saw his eyes is more than likely a bad camera angle or missed mark by Burton, or his head was at a bad angle.
Blaze