Skawo reports on an odd bug in both the Capcom-made Gameboy Advance releases of Zelda games A Link to the Past and The Minish Cap. It’s explained, as is frequently the fashion, in a ten minute Youtube video, here:
The video’s a bit padded with injokes and gimmicks, but beneath it all the bug is really interesting. Many games have checks to ensure the validity of save data, but the developers of both games implemented theirs in an odd way, calculating a 16-bit checksum for the file data twice, once by adding and once by subtracting, saving them both, and them when the File Select screen is setting up adding them to each other and checking for zero with the negative bit set (the high-order bit). It usually works, except when the checksum is exactly zero, which happens one in 65,536 times.
When that occurs, the total will be zero without the negative sign, which will be detected falsely as corrupted save data. As luck would have it, naming your character “God” in the European version of GBA Link to the Past will trigger the bug, and make it so you can’t create the file. But the 1-in-65536 chance comes up every time you save and exit. (The file check is made upon loading the File Select screen, so just saving with a checksum of 0 won’t trigger it; if the player saves later in the same play session, non-zero checksums will be written over the bad ones.)
1-in-65536 is a rare event, but it’s not extremely rare, and it’s absolutely the case that over the years many players have had their games declared corrupted and made unloadable. If a player saves their game, say, 20 times through a playthrough, then that’s about a 1-in-3250 chance of losing all their progress, and both games sold much more than 3,250 copies.
Skawo demonstrates the bug in action in The Minish Cap here, in the peocess responding to some naysayers (10 minutes):
