Oh man, funny story, this just happened to me a few days ago. I feel asleep, I guess, on top of my left arm. When I woke up at 6 AM, my arm was completely numb... However, I had some sort of phantom limb thing and it felt like my arm was fine. I rolled over, and somehow my arm spasms or something, flails upward, and drapes itself across my body.
All of a sudden there is this heavy, hairy arm lying on top of me, that I have no control over. Two thoughts instantly rushed through my mind... The first: "Woah, there's somebody in my bed." The second, because I play too many video games, "Fucking ZOMBIES!"
For a moment, I was absolutely terrified. Now, I don't know why, but for some reason, my instinctive horror scream was, at the top of my lungs, yelling: "WHAT THE FUCK?!"
Shortly after, I regained feeling in my arm, realized how stupid that just was, and went back to sleep.