Getting called out by my name in the literature club, or being told about how I could have used my superpower better in that other game. Two get-up-and-take-a-walk moments for me.
On a different tack, the introverted but social Unreal Tournament clan nights, Instagib with Relics. Played that to absolute death, clan nights with pizza and beers. Worms, too, with four people sharing a keyboard.
Lands Of Lore, Eye of the Beholder, BG1 and 2, etc. when solving a puzzle or finding a secret after being lost for hours.