In Case You Missed It: Uncle Steve’s Letter (StarTropics 1990)
(Published May 30th 2016)
Stapled with the game’s manual was this letter which was needed to progress in the game. At a certain point in the game, the main character (Mike) is asked to use a code that is hidden in the letter which he received from his uncle. The player (in real life) needs to submerge the letter in water, revealing the code, and essentially turning it to pulp. This made progressing through the game rather difficult for kids who rented it.