Probably my best "oh stuff" moment was a good one in Half-Life 2: Episode 2, [spoiler]when you're doing the mission where you and a Vortigaunt have to go down into the antlion nest underground to get the nectar to heal Alyx, the Vortigaunt and I were in the part with the elevators and you have to fight off a stuffload of zombies and antlions while waiting for the elevator to get there. This zombine (combine zombie) went to attack the Vortigaunt and the Vortigaunt grabbed the zombine's hands and threw him down on the ground where he proceeded to curbstomp it.[/spoiler] I was like "forget yeah."
Worst "oh stuff" moment was when a friend and I were playing gm_ghosthunt2 in Garry's Mod. I was in a somewhat small room looking for stuff when the door closed behind me, trapping me inside with my friend on the outside, unable to get to me. Obviously I said "oh stuff" when the door closed. What happened next is probably as close as I've ever come to a heart attack. There was a big desk in front of me in the room that, about 10 seconds after the door closed, launched itself off the floor and into my face, slamming me against the wall. I then heard my friend shout, "OH GOD" because apparently, as I learned shortly after, at the same moment when the desk hit me, a floating black figure with red eyes appeared out of loving nowhere right in front of my friend and proceeded to scream at him for a couple seconds until disappearing. The door then opened and we both went back to the safe area to clean the stuff out of our pants.