Season 2: Episode 21 - The Vegas Renormalization

The Vegas Renormalization: When Wolowitz is dumped by Leslie Winkle, the guys take him to Vegas. He hides in his room the whole time looking at Leslie's Facebook page. The guys finally lure him out with a prostitute. In the mean time, Sheldon stays back home and gets locked out of his apartment. He ends up spending the night at Penny's place.