Metal Gear Solid · PlayStation · 1998
A fourth-wall puzzle whose only solution — a radio frequency — is printed on the back of the game’s physical packaging, not anywhere in the game itself.
Hideo Kojima built Metal Gear Solid around breaking the boundary between game and player, and the Meryl codec puzzle is its slyest example. Early on, Snake needs to contact Meryl by radio to progress, but no character ever tells him her frequency and it appears nowhere in-game. Colonel Campbell instead instructs the player to "look on the back of the CD case" — where a screenshot of a codec call clearly shows the number 140.15. Players who had thrown away or lost the packaging were genuinely stuck, forced to seek help. It sits alongside the game’s other reality-bending trick — the Psycho Mantis fight, where the villain reads your PlayStation memory card and asks you to switch controller ports — as a defining moment in games treating the player, and the physical product itself, as part of the fiction.
When Campbell tells Snake to check the CD case, look at the photo of a codec conversation printed on the back of the game’s packaging; Meryl’s frequency, 140.15, is visible in it.