Leonard gets stuck in the middle of Sheldon and Penny's fight
Leonard got stuck in the middle of an argument after Penny accidentally shot a paintball gun at Sheldon's couch.
They later battled it out in an actual paintball game where Sheldon killed Penny because of her cushion incident. Since the blonde waitress couldn't shoot, Leonard killed Sheldon, who then broke the rules, retaliated, and killed Leonard. Luckily, the trio ended up forming a truce in "The Cushion Saturation" (Episode 16, Season 2).