Lagging and kids. I don't generally like kids anyway, but the worst example I can think of was this idiot child who had just turned 3 (I know this because the kid would shut the fuck up about it), and tried to play on her brother's computer. The game that happened to be running at the time was TF2. And I just happened to be in that match. First off, one guy's skill level dropped drastically and then he started switching between the different classes schizophrenically. Then the idiot child opened her mouth. And wouldn't stop going on about her personal life and stating the very obvious about what was going on in the match. It was worse for me because the little girl had decided to play as a spy at one point (my favored class), and started following me around (I was the only other spy on our team), and kept shouting things like "We're swimming! Yeah!", thus giving away our position... just a pity that friendly fire was disabled...).