On the left, notice the jukebox glass is fogged over. On the right, look closely at the where Nip-It's backglass should be.

However, in this shot, nothing wrong with the backglass. Notice, however, the lack of the lock on the coin door.

Maybe Arnold owns the game, which might explain why Fonzie is working on it, instead of the operator.

Maybe Fonzie magically transformed the backglass somehow?

Maybe somebody on the set broke the original?

This even made the Fonz lose his cool.