Season 1: Episode 16 - The Peanut Reaction

The Peanut Reaction: The gang finds out that Leonard's birthday is coming up. Penny wants to throw him a party, but she finds out that his family never celebrated his birthdays. Penny decides to throw him a surprise party and enlists Sheldon to help her out. Wolowitz participates by taking Leonard to the emergency room with him, under the pretense that he ate a peanut and is having a massive allergic reaction. By the time they make it back the apartment, the party is over. Penny feels terrible and gives Leonard a big kiss.