Bioshock had one great moment, I was walking I saw the shadow of a splicer, i was ready for a fight, the lights black out, when they turn on the splicer has left, I then relax obviuosly the splicer saw me coming and ran for it. so I casually radi the room, I was ready to leave when the Splicer jumped out, I panicked (In real life i was thinking HOLY S#!T) and was firing the automatic like crazy.
I'm most surprised when I'm not on edge, simply focusing on a goal, when something comes out of nowhere like a loud noise, it disrupts my calm.