The Big Bang Theory Season 2 Episode 7 : The Panty Piñata Polarization
When Penny's cable gets cut, she starts coming over to the guys' apartment to watch America's Next Top Model. However, Sheldon bans her from the apartment for a series of petty infractions, such as touching his food, sitting in his spot, and sending him "email humor" about a cat "who wants to haz cheezburger". After a series of escalating pranks, including Sheldon cutting off Penny's access to WiFi and Penny wrecking Sheldon's "laundry night", Sheldon goes to the extreme of hanging her laundry from a telephone wire outside his apartment. Leonard decides to help Penny end the war, and gives her Sheldon's "Kryptonite": his mother's phone number. Subsequently, Sheldon's mother calls him and forces him to apologize to Penny.