Got a couple here, one online and one Multiplayer.
Online: Playing DDO as a rogue, every about 7.26 seconds I said, "careful, its a trap." Anytime anyone told anyone else to do something, I said, "Don't do it, its a trap." After about 45 minutes of this, someone in the group says, "Dude, seriously, shut the hell up about traps already." Soon after, the party blunders into a trap and all of them but me are killed. When the guy who was a little short on patiences says, "Damned rogue, you should have warned us about the trap!" I couldn't help but point out, "But you told me to shut up about traps."
Multiplayer: Sat down for the first time ever in front of a 360 and my friends were playing Soul Calibre. My turn comes up and as its doing the preliminary taunts and whatnot, I'm slowly and lowly saying "Kaaaaaaaa-maaaaaaaa-yaaaaaaa" and just as it starts, I shout, "THIS BUTTON!!!" at the top of my lungs and proceed to do some hellacious combo. I still lost, but it was a fun first bout.
Less of a funny quote but still fun. Friends wanted to do all shotguns in a Halo 3 match. I tend to dominate our shotgun matches, and was doing fairly typical in this particular situation. Well, I hear a "whoop!!!" from the other end of the room as my buddy shouts "Finally killed Draygen! Who's the bad ass now?" As he turns to my chair to gloat, he sees my 3 year old son sitting in my chair with the controller mashing random buttons as I'm returning from the kitchen with a sandwich. I still won that match, and he felt like a total prick for killing a 3 year old.