In Pokémon Stadium 2, as it was released in the West, the mini-game “Delibird’s Delivery” can deliver candy, stockings, Game Boys, Nintendo 64s, Snorlax dolls, grand pianos, and diamond rings. If you search for videos of “Delibird’s Delivery” on the Web, you’ll see countless games played with these seven presents and no surprises.

Screenshot of Delibird’s Delivery from an English version of Pokémon Stadium 2, showing the seven available presents.

But if you search by the mini-game’s Japanese name, “はこんでデリバード”, you’ll quite often see two different presents: a Game Boy Advance in place of the Game Boy, and a GameCube in place of the Nintendo 64. Either of these presents would be quite a surprise for a child to unwrap on Christmas in 2000, the year that Pokémon Stadium Gold & Silver was released in Japan, as neither system would be released until the following year. An anonymous commenter on the site “1990ちゃんねる” tells us of their confusion:

ゲームキューブが表示される理屈が子供心にわからなかった
64に新ハードの情報ってどう入るの？みたいな

As a kid, I never understood why the GameCube shows up.
Like, how does the N64 get the news about new hardware?

And that’s an excellent question to ask, because there was a way for Pokémon Stadium Gold & Silver to receive up-to-date information: through Pokémon Crystal Version and the Mobile System GB. Indeed, Mobile Stadium downloads included two flags for precisely this purpose: one to unlock the Game Boy Advance and one to unlock the GameCube. The developers had planned ahead so that they could activate the first flag in March 2001 when the Game Boy Advance was released, and the second in September when the GameCube was released.

Screenshot of Delibird’s Delivery from a Japanese version of Pokémon Stadium Gold & Silver, showing the GameCube and Game Boy Advance among the seven presents.

However, the Mobile System GB was only used by a small fraction of Pokémon players, and Mobile Stadium by a fraction of that fraction. If this was the only way for these presents to appear, they shouldn’t be anywhere near as common as they apparently are. And someone by the name of PokeNas, a contributor to the REON Team aiming to recreate the Mobile System GB, dealt a double blow to this explanation: they told me that their copy of the game had only the GameCube appear, not the Game Boy Advance, which makes no chronological sense, and that it could not have received data from Mobile Stadium because the menu option was never activated.

Some Japanese sources have vague and unverified theories about how these presents can be unlocked. Some wikis, including the Japanese Wikipedia, assert that they are unlocked when using the Transfer Pak.[citation needed] A YouTube commenter, after being challenged on this received wisdom, replied that they must have unlocked it themselves by completing the Gym Leader Castle. Another source says it requires playing Delibird’s Delivery many times, but doesn’t specify how many. But none of these theories would explain why the presents don’t appear in non-Japanese editions of the game.

The truth is that the Game Boy Advance and GameCube presents are exclusively unlocked by Mobile Stadium, which is why they don’t appear in versions where Mobile Stadium was removed. But the game’s code mishandles Mobile Stadium data in a way that can cause one or both presents to be unlocked without ever actually using Mobile Stadium.

When a Game Boy cartridge is first manufactured, or when its internal battery is replaced, the contents of its save data are uninitialized and effectively random. When the game is saved, it has to put some sort of pattern into its data that is unlikely to occur by chance, so that it can distinguish actual save data from random noise. In the case of Pokémon Crystal Version’s Mobile Stadium, the pattern is that the data must end with the characters “P3” followed by a checksum. If “P3” doesn’t appear where it should, or if the checksum is incorrect, then Pokémon Stadium Gold & Silver knows there’s no Mobile Stadium data. If both “P3” and the checksum are correct, then it can safely assume that Mobile Stadium data was written deliberately.

Because the Transfer Pak is fairly slow, most of Pokémon Stadium Gold & Silver’s game modes only load the specific blocks of data that they need from the Game Boy cartridge. For example, Event Battle needs the players’ six party Pokémon, but it doesn’t need any Pokémon from PC boxes or any data from Mobile Stadium, so it doesn’t load those. But there are a few parts of the game where the developers didn’t bother to pick and choose which data they needed; they instead just read all the things. And if any of that data needs to be updated, they just write all the things back to the Game Boy cartridge. And in the process of writing all the things, the game writes the characters “P3” at the end of the Mobile Stadium data and recalculates the checksum, ensuring that the data is treated as valid even if it wasn’t before.

I haven’t identified all of the game modes that do this, but one of them is My Room (also known as Your Room, because pronouns are hard), the mode that shows room decorations from Pokémon Gold, Silver, and Crystal Versions. If you go into My Room with a Crystal cartridge, it will load Mobile Stadium data along with all the other data. If you change any decorations, and save your changes, the Mobile Stadium checksum will be recalculated. If you go into My Room with that cartridge a second time, the game will confidently assume the Mobile Stadium data is valid and look for the flags to unlock the Game Boy Advance and GameCube in Delibird’s Delivery. If your Crystal cartridge has never downloaded Mobile Stadium data, and has never had its save data cleared with Up + Select + B, then it should still contain the same random data that it did when the battery was installed. Theoretically, that random data has a 50% chance to unlock the Game Boy Advance and, independently, a 50% chance to unlock the GameCube. Here’s a demonstration:

Because this glitch is so easy to trigger through normal gameplay, it is difficult to know if these flags were ever officially enabled through Mobile Stadium. Did the developers learn of the glitch and decide to abandon the feature? Or did they enable it quietly without drawing attention to the mistake?