Season 3: Episode 3: The Gothowitz Deviation

Wolowitz meets a Goth girl online. When she asks to meet at a local Goth club, Wolowitz takes on his own Goth persona in order to impress the girl. Wolowitz and Koothrappali go to the club to meet her and her friend. The girl seems into Wolowitz and suggests they go get tattoos together. Wolowitz, thinking he is going to score, agrees to go. But all goes bust when Wolowitz chickens out of getting a real tattoo.

Sheldon is trying to use positive reinforcement to train Penny to behave how he likes. Leonard catches on and initially is appalled by it, telling Sheldon to stop. But when Leonard sees how well it works, he decides to use it on Penny to achieve his own objectives. Penny catches on and gives Leonard a little negative reinforcement.