It's possible, but very, very rare. Most spider wouldn't be crawling on a bed, as most house spiders like the floor (especially places that aren't disturbed very often, like the basement). Spiders also don't like to walk on moisture or in windy places. So they wouldn't be likely to go into a mouth with moisture and breathing. One other thing is that even when people are asleep, I don't know how easy it would be to swallow a spider that doesn't want to be swallowed.