"Take your word, thief."

    Backtracking became possible thanks to the MMC1 chip contained in the Metroid NES cart, which allowed the relatively lightweight NES hardware to do new programming tricks such as reverse scrolling. Another noteworthy technical fact is that Metroid, in Japan, was a title for the Famicom Disk System, which allowed game data to be saved between playthroughs. The US was denied the FDS, and developers came up with various alternative solutions, such as passwords and battery saves, to store game data. Although later games used relatively intuitive password systems such as grids of game-related symbols, Metroid saddled players with long, meaningless alphanumeric strings. Transcription errors abounded, as legions of kids learned the hard way to differentiate between "0", "O", and "o". Unlike later password systems (such as Megaman 2's) the Metroid system was relatively unhackable, meaning players could not change parts of their passwords to improve their stats, with the exception of the infamous "JUSTIN BAILEY" password, which allowed players to use Samus in her female form, complete with a wide range of attacks and enhancements.