Forbidden Four

The "Forbidden Four" is the name given to a group of NES games in that cannot be obtained through the use of universal codes. The four games are Mario Bros., Super Mario Bros., Ice Climbers, and The Legend of Zelda. The moniker started out as the "Forbidden Five" (the fifth title being Punch-Out!!) and referred to the five NES games that at the time could only be acquired through the use of a cheating device. The accidental generation of a Punch-Out!! code by GameFAQs user darklao on August 12, 2003 provided access to that NES title until the game's code generator was cracked completely by Ryan Holtz in December that year. By using a disassembler for the GameCube's CPU, Holtz was able to examine the game's low-level code pertaining to code generation, and his findings allowed him to create an application to replicate the codes generated by the game. Furthermore, it was determined that the developers had specifically excluded the Forbidden Four from being distributed using passwords; the Forbidden Four were meant to be distributed by other means.

Trivia

 * In North America, Mario Bros. and Ice Climber were released exclusively through the use of E-Reader cards. Europe never received e-reader support, so these games cannot be obtained in that region without an Action Replay. Japan never received the cards either, but these games were available in Doubutsu no Mori + and Doubutsu no Mori e+. For a short time, Nintendo of Japan offered to transfer equivalent save data from Nintendo 64 to GameCube, so it can be played legitimately, however, this service has since been discontinued.
 * The only games that cannot be obtained at all through normal means are Super Mario Bros. and The Legend of Zelda. Interestingly, Super Mario Bros. can be played via Advance Play, and Doubutsu no Mori + includes a port of the Famicom Disk System version of The Legend of Zelda. This is odd as the English release replaces that with the English NES version of it, even though the game is completely inaccessible. This led some to believe that these two games either are obtainable through an undiscovered universal code or were originally intended to be unlocked via e-Reader as well, but e-Reader support died off before it could happen.
 * Some NES titles feature Advance Play, which allows the player to link a Game Boy Advance to the GameCube and download the NES game to the handheld temporarily. This feature is not available for games that were originally produced for the Famicom Disk System, such as Clu Clu Land D and The Legend of Zelda. It is also not available for games larger than 192 kibibytes, such as Punch-Out!! and Wario's Woods, which cannot fit into the Game Boy Advance's RAM. All other games can be played on Advance Play, but their graphics are stretched by about 17% on the screen's display and are limited to one player.