Sheldon plays Words with Friends against Stephen Hawking and becomes obsessed with winning, only for Hawking to play a low-scoring word ("act"), leading Sheldon to discover that Hawking is a "sore loser" who intentionally slows down the game when losing.
Fans often debate Leonard's behavior in this episode, specifically his decision to rewrite Penny's paper behind her back. [S6E6] The Extract Obliteration
Hawking game or perhaps a breakdown of in later seasons? Sheldon plays Words with Friends against Stephen Hawking