oh_riginal said:
Maybe when he sees what Vader looks like, he doesn't like it and puts the mask back on immediately.
He could take the mask off, scream NOOO (like he did in Episode V), and immediately put the mask back on Vader.
It could also be funny if Obi-Wan told Luke that Yoda was his father. Or Luke could think the real Yoda was his father. Maybe that would be more creepy than funny--I don't know.