It was either Doom 3 or Quake 4, I can't remember.
When I was in school several years ago I used to have a habit of playing video games at all hours of the night. So there I was, playing at like 3 in the morning with all the lights off and there was a noise that sounded as if it were behind me. I turned around and looked behind me and of course there was nothing there, so I kept playing... took what had to have been 5 or 6 steps forward around a corner and this ungodly monster jumped out at me.
Scared the life out of me, that's for sure.