The Shiny Trinket Maneuver - S5-E12
Corrected entry: After Amy stalks off on Sheldon, Penny tells him off because she told him some good news about her paper being published, and he belittled her achievements while bigging up his own, trivial activities. She is very specific about what he did and why Amy reacted. How did she know all of this? She was nowhere near the table at the time and could not possibly have overheard their conversation. She might assume Amy is mad at Sheldon but she can't possibly know why.
Correction: Penny is behind them cleaning a table and you actually see her overhear what they are talking about.
Anastasios Anastasatos