The Japanese Bonus Disc allows players to get Celebi, but for the American release, Jirachi was made available instead. PAL areas, however, didn't get the Bonus Disc at all (Jirachi being made available via Pokémon Channel instead). Players in both regions who wanted Celebi had no luck at all.
Japanese versions of Pokemon Colosseum originally supported the GBA e-Reader and several cards; however, this support was removed in localization, cheating players out of, namely, a Shadow Togepi, Shadow Mareep, and Shadow Scizor.
The only species snaggable in both games, not counting Togepi and Mareep, which were E-Reader events in Japan, is Makuhita. However, many families are in both: Swablu/Altaria, Houndour/Houndoom, Slugma/Macargo, Ledyba/Ledian, Piloswine/Swinub, Pineco/Forretress, Teddiursa/Ursaring, Spiarak/Ariados, and the Hitmons. Others can be obtained in both via trade or poke spots (in XD).
A shiny Pokémon from this game is one of the rarest shinies in the entire franchise. Shadow Pokémon can be shiny but it is impossible to tell until they have been captured whether they actually are. note The Original Trainer ID of the Pokémon changes when caught, and the Original Trainer ID is used when calculating whether the Pokémon is shiny. Any opposing Shadow Pokémon that appear shiny are almost certainly going to revert back to regular coloring once actually checked.