The Infinite Loop That Wasn't: A Holy Grail Bug Story - mGBA

Once upon a time there was a game for the GBA by the name of Hello Kitty Collection: Miracle Fashion Maker. It was a cutesy game based on the famous Sanrio Hello Kitty franchise, developed by Access. But hidden beneath the surface of the seemingly innocuous title was an insidious problem. Somehow, this simple game just did not boot in any GBA emulator. This alone was enough to qualify it as a Holy Grail bug. As with all Holy Grail bugs the bug itself was absolutely baffling. Here the explanation was simple: sometime during the game’s startup sequence, it enters a loop that can never exit due to expecting a specific value reading out of memory that does not exist. While many games have similar bugs, such as the intro sequence in the high-profile game The Legend of Zelda: The Minish Cap, those rely on a specific behavior from reading invalid memory addresses. But this loop seemed to defy that behavior. And yet, it works on actual hardware. Moreover, this same bug also occurs in when loading a save in Sonic Pinball Party after a cold boot. Could the expectation of these invalid memory accesses be wrong somehow? But how?