A lot. Robert Pirsig, author of the seminal 'Zen And The Art Of Motorcycle Maintenance', had a mystical experience, after which he was carted away to a mental institution and had his brain fried with numerous doses of ECT (now illegal). It took him several years afterwards to figure out who he was and why he was there. He was there because well-meaning but misguided people didn't understand what had happened to him. Its the ultimate irony: he had discovered the true nature of reality (which he calls Dynamic Quality, but which I would call God) and the first thing people did was to try to bring him back into the world of illusions, or kill him trying.
He had to pretend he was sane to get out. Interestingly, he said that the insane say that 'sane' people are all actors. Having given up the act themselves they can spot a phoney a mile off.
Incidentally Pirsig's Metaphysics of Quality (found in his second book 'Lila', which I urgently recommend to any materialist) has been called 'The high point of western philosophy'. Not bad for a nutter.