Oh, sentient NPCs, my biggest irrational fear.
That feeling when you're working on a game, it's 2AM, the only sound you hear is the computer fan, and suddenly you see something that looks out of the place in the corner of your eye.
I once jumpscared myself in the editor. I was working on something on one side of a map, and then i saw two glowing eyes starring at me through a door frame. Turns out it was a car with lit headlights i added to the other side of the map a day prior.