The Cake is a Lie is a catchphrase popularized by the game Portal, and is often used to convey the message that a promised gift is being used to motivate without any intent of delivering.
In Portal you are required to do numerous tasks in a research facility guided by a computer named GlaDOS, who repeatedly mentions that following the experiment, there will be cake.
As the game progresses, the player finds messages written on the walls as warnings from past test subjects. One message scrawled on the walls in later levels reads “the cake is a lie. the cake is a lie. the cake is a lie. the cake is a lie”.