For a certain reason, Kana and Chiaki are playing a game that Kana has borrowed from a friend. We see the screen of the TV as they play the game. Chiaki is controlling "sensei" and Kana is controlling "Ninomiya-kun", a high school girl.
They walk into a room. The high school girl runs into a crate and stalls, running in place. A zombie comes out of the side door and starts attacking the girl. She screams ("kyaa!"), still running in place, and it attacks her again. She screams again, suddenly fires two rounds from a gun she hadn't had a moment before, and then dies. "Ninomiya-kun!" says sensei.
This is the funniest moment in the series so far, at least to me. Why? Because it was so real. The element of truth was what made it so funny: Kana is playing a game she's never played before, and she doesn't know how to play it. So her character fumbles around on the screen and gets killed in very short order: "Kyaa! Kyaa! blam blam Eeyaiggh! Ninomiya-kun!"
Back in 1984, Epyx released "Summer Games", which was an Olympic-themed game in which the goal was, obviously, to win the gold medal in various events. One was cliff diving.
The first time we played the game, a friend of mine decided to try the cliff diving game. He hit the button on the joystick; the character on the screen jumped off the top of the cliff and proceeded to fall into the water like a stick: stiff and straight as a board, rotating around his center of mass:
It was the funniest goddamned thing I'd ever seen on a computer up to that point in my life. I was completely helpless with laughter. And even now I laugh out loud at the memory of what that hapless diver looked like as he fell.
That's why the scene in Minami-ke is so funny to me; I remember trying to play computer games without reading the manuals, and the resulting stupidity.