Well there would be several online moments between people, but I assume you mean actual in game moments, so here's a pretty good one:
Long ago when I was playing Bio Shock, I had to face my first "Big Daddy". Obviously I wasn't to happy about the situation. In fact, I cried a while, and I might have peed a little. But when the time came to man up and fight this thing, I charge in the room guns blazing. They weren't to happy about that. The Big Daddy ran towards me and gave me a nice bonk on the noggin, and continued to pummel me into the ground (all while the 5 year old girl was screaming for blood). Scared for my life, I make a run for it into another room. As i run, I notice that the thunderous footsteps are fading. I run back to the door between the room I was in, and the room I was previously being clobbered. It seemed this 2 ton hunk of metal was having trouble getting through a wooden door frame. He just kept charging it, beating it, and drilling it.(well virtually what he was doing to me earlier, but this time with no success) I stand there for a few minutes, waiting for him to burst through. But it never happened. which was pretty amusing, I could shoot, smack, and zap him to my hearts desire. All because of this divine door frame that God had sent to save my sorry ass.
EDIT: Whoops, didn't see that little poll there. Still, funny moment.