In 'The Staircase Implementation' Leonard told Penny about how he came to live with Sheldon and how the elevator came to be out of order. Viewers experienced the story through a series of flashbacks to 2003. Leonard moved in with Sheldon despite being warned of Sheldon's craziness by his former roommate as well as Louie/Louise, the transvestite who used to live in Penny's apartment.
We also saw Leonard become friends with Howard and Raj. Leonard had some rocket fuel in the apartment that Howard needed for an experiment. While using the rocket fuel, Leonard made a calculation mistake and needed to get it out of the apartment. He thought he had lots of time, but Sheldon knew otherwise. He grabbed the container from Leonard, pushed Leonard out of the way and sent the rocket fuel down the elevator. It exploded in seconds. While it was technically Sheldon who broke the elevator, Leonard admitted he was at fault because he had the rocket fuel and he made an error. He revealed to Penny that he stuck by Sheldon over the years because Sheldon saved his life and didn't rat him out to the landlord or to Homeland Security. Penny was annoyed that Leonard was the reason she had to use the stairs every day.