Well, something actually happened to me, while I was in a deep sleep.
It's not the "worse thing" that could happen to a sleeping person... but it was a very miserable startling awakening to say the least.
To borrow the worse opening line of a novel ever: It was a dark and stormy night (literally!). One night while I was camping out in the mountains, unfortunately a huge/heavy thunderstorm and massive downpour hit that area.
Unfortunately I had made the foolish mistake earlier that night of pitching my tent in a water-run-off channel.
When I set up the tent initially, it was of course dry. It just seemed like a really nice flat clearing to pitch a tent.
But boy was I wrong.
Because I had been hiking and burning so many calories earlier that day, I had actually fallen into a very deep, hard sleep, that night, so I did not wake up initially.
When I did finally wake up, there were several inches of water in my tent.
My hazy mind was like, WTF?!
And then as I gasped I inhaled a nice big mouth full of muddy water, and began choking. I rose to my feet instantly in panic, feeling like I was drowning, and coughing up a storm to clear the muddy water from my lungs, as I splashed/thrashed around my water soaked tent.
And of course, as luck would have it, that night was also COLD, despite being summer time, and everything I had, all my clothes, my sleeping bag, were soaked/drenched in cold water.
I immediately stumbled outside to stand in a shallow fast moving river, illuminated with flickers of lightning.
I rapidly tried to take down the tent in the middle of this river, but my fingers were going quickly numb, and curling up from the cold, and my hands were shaking/shivering something fierce.
So I grabbed a bunch of clothes, jammed them into my backpack and raced to the shore.
I knew I had to get to warmth, so I was stumbling down the mountain, falling, tripping, as huge amounts of rain continued to pour down... with lots of lightening flashing like a strobe.
I finally made it a camping-park below, and stumbled into the public bathroom. I held my fingers under running hot water, and then spent at least 4 hours drying my clothes under the hand-warmer dryer in the bathroom.
Sucky way to wake up, for sure!