I dunno, I went to a high school reunion last year, and a dude seemed to know people's phone numbers from 30 years ago.

If he could remember that sort of thing, I can believe there are people who can remember steps of a proof, which is a much less random thing that you can feel your way around, given a few queues from memory.

Plus, realistically, how closely does an examiner read a proof? They have a stack of dozens of almost the same thing, I bet they get pretty tired of it and use a heuristic.