This only happened to me once, I think I'd just smoked way too much. It was actually okay when I was sitting down, but when I stood up every single muscle completely spazzed out, my vision clouded over and I fell over shaking and could barely see or move for a few minutes, I just had to lie there with my muscles going mad. More often, my heart just beats way too fast and I can't stay standing for more than a minute or two while I'm really stoned or once again, my vision blacks out. I've never actually lost conciousness this way, but it's not much fun.