The Big Bang Theory Episode 2.01 The Bad Fish Paradigm

After Penny and Leonard's great date, Leonard tries to kiss Penny in the hallway and realizes that the guys have secretly rigged cameras to spy on them. Penny then tells him that they should take things slow. We find out later when Penny confides in Sheldon that she is actually insecure about not being smart enough for Leonard. She asks Leonard to keep this a secret - but he is terrible at keeping secrets. After some warm milk laced with valium, he spills the beans to Leonard. Leonard tries to convince Penny that it is not a big deal, but it backfires, and Penny does not want to see him anymore.