Falling or slipping off something very high up. In the next few seconds, you know you are going to die.
I remember hearing a story about how a father and daughter were hiking up some mountains and the cliff they were standing on gave way and they both fell to their death.
Thinking about how you would feel when you're falling and all of the things you were going to lose in the next few seconds, and the thoughts you would be having.. and the people you were going to leave behind terrified me.