The Big Bang Theory Season 1 Episode 3 : The Fuzzy Boots Corollary
When Leonard sees Penny kissing a man in front of her apartment door, he is devastated thinking she has "rejected" him. The guys convince him to ask someone out at work, so he asks out fellow scientist Leslie Winkle (Sara Gilbert), who preemptively turns him down. As Leonard slips deeper into his depression, which involves thinking about buying a cat, Sheldon points out that he was not actually rejected by Penny, as he had not asked her out. Leonard agrees and promptly asks Penny out to dinner, but she mistakes his invite as an invitation to hang with all the guys. Despite being aware of this, Leonard keeps the date and makes up excuses as to why the guys were absent.