I think it happened to me once. I woke up on my right side and there was this very real feeling that something or someone was in bed with me, hugging me from behind. Then i realized that i couldn't move at all and started to violently shake my head in an attempt to move or wake up or figure out what the fuck was going on.
I think i just went back to sleep after that...it may have been a dream though.
Speaking of frightening things that happen when you sleep, anybody else had dreams where you're just scared shitless for no reason whatsoever and wake up drenched in sweat even though there was nothing particularly scary about that dream? And no, i haven't had any traumatic experiences...that i know of.