![2girls ahoge blue_eyes box capelet cardboard_box character_request closed_eyes comic commentary eating english_commentary gift gloves highres hodaka_misogi long_hair madvalentine89 multiple_girls o_o orange_hair paper parody pecorine portal_(series) princess_connect! princess_connect!_re:dive the_cake_is_a_lie rating:s score:8 2girls ahoge blue_eyes box capelet cardboard_box character_request closed_eyes comic commentary eating english_commentary gift gloves highres hodaka_misogi long_hair madvalentine89 multiple_girls o_o orange_hair paper parody pecorine portal_(series) princess_connect! princess_connect!_re:dive the_cake_is_a_lie](/cdn_image/preview/8d/26/8d2629ae6266408eb5d5093436c683b0.jpg)
Popularized by the game Portal. During the game, GlaDOS encourages you to solve intricate puzzles using cake as a motivating perk. When you have "broken out" of the game's initial testing phase (from threat of death), you find scrawls on walls of the innards of the testing center warning you that "the cake is a lie".
It can be used as a phrase to impliy that a promised reward for one's work is false, with the one offering the reward never intending to give it in the first place, possibly because the reward never even existed.