Metroid · NES · 1986
A password that starts the player near the end of the game with most power-ups collected, and crucially, displays Samus Aran as a woman in a swimsuit rather than in her Power Suit — one of the first times a major game protagonist was revealed to be female.
Metroid used a password system rather than battery-backed saves, and the password space was large enough to produce many accidental or intentional shortcuts. "JUSTIN BAILEY ------ ------" (with spaces filling the remaining slots) unlocks a late-game start with extensive power-ups and Samus depicted in a leotard rather than full armour. The origin of the name "Justin Bailey" has never been definitively explained — some sources suggest it was a Metroid programmer's name, others that it is a coincidence in the password algorithm. Regardless of origin, its discovery transformed player understanding of the game's narrative: the post-credits reveal of Samus as a woman was already a shock, but the Justin Bailey code let players know the secret before they even began.
At the password entry screen, enter: JUSTIN BAILEY followed by twelve spaces (or dashes, depending on region). Press Start. The game will begin in Norfair or Tourian depending on exact input, with Samus unarmoured and carrying most major power-ups.