The Big Bang Theory Season 5 Episode 18 : The Werewolf Transformation
Sheldon refuses to let anyone cut his hair except for his regular barber, even going to see his barber in the hospital and trying unsuccessfully to talk him into coming out of his coma. Penny offers to trim his hair because she use to cut her brother's but he declines. Penny later tells him to embrace the chaos and that it just might be fun not knowing what will happen next. Sheldon agrees to her proposal and decides to wear his Tuesday pajamas on Thursday night and takes up playing bongos in the middle of the night. After objections by Leonard and Penny he ends up on Amy's couch. With Amy's vote of confidence, Sheldon agrees to Penny cutting his hair, which she messes up at the end of the episode, unbeknownst to Sheldon.