The Big Bang Theory Season 1 Episode 3
The Fuzzy Boots Corollary
When Leonard witnesses Penny kissing a guy in front of their apartment, he is hurt and feels "rejected." The guys try to convince him to ask a girl out at work and when he's rejected by Leslie Winkle, he is even more depressed. He goes so far as to think of buying a cat. Sheldon, in an attempt to avoid getting a cat, tries to convince Leonard he was never actually rejected since he never asked Penny out.
Leonard goes to ask Penny out to dinner, but she mistakes it as a group date with the guys. Leonard makes up excuses for why the guys are unable to go and the two have the date alone. Penny explains that the guy is not actually her boyfriend and just a weekend long fling she's using to get over her ex. Leonard accidentally inures himself ending the date short. Leonard is unable to build up the courage to tell Penny how he really feels.