The Big Bang Theory Season 1 Episode 10 : The Loobenfeld Decay
Penny is given a part in Rent, though after hearing her sing extremely off-key Leonard lies to avoid seeing her performance so he won't hurt her feelings. Sheldon becomes worried that Leonard's lie was not good enough, so he creates an intricate lie about his non-existent drug addicted cousin "Leopold Houston", who was in need of an intervention (and thus why they could not see the play). Sheldon becomes so obsessed with the credibility of his lie, that he has Toby Loobenfeld (DJ Qualls), a research assistant who double majored in Physics and Theatre, impersonate cousin "Leo" to convince Penny that the lie is true. In the end, not only does Leonard still have to see the videotape of Penny's performance, but Penny and "Leo" start flirting as well.