Only once that I can remember vividly. I was getting out of bed in my dorm in college, which, of course, was on cinder blocks. And pow, straight to the floor as soon as I tried to get out of bed. It was pretty scary actually.
i pinched a nerve in my back recently and it makes my right arm and right leg fall asleep a lot. the other day i woke up and thought i had a stroke because i could only feel my left side.
Once, after a long nursing session/nap on the couch. I stood up with the baby, took two steps and almost collapsed. I just tried to aim myself over to the loveseat and not squish the baby.