What was the most horrifying experience you've ever been in?

Sleep paralysis: after arriving in a foreign country exhausted and having a long sleep, I woke up paralyzed. I could not move or utter a sound. Now I know that this is a thing that can happen when your sleepy mind and your body are momentarily out of synch, and when I've had similar experiences (fortunately not often) I did not panic. But at the time I had no idea what was happening to me.