the cake is a lie

Origin: From the video game Portal (2007), in which a self-aware computer, GLaDOS, lures the player into participating in dangerous experiments with the promise of a cake. As the game progresses, the player discovers graffiti left by the only previous test subject to survive GLaDOS' murderous scheme, including the warning the cake is a lie. Origin information sourced from Wiktionary contributors (CC BY-SA 4.0).