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Bad eggs (Japanese: ダメタマゴ) first appeared in the advanced series of Pokémon games. It is the typical result when data for a Pokémon is corrupted, either by the user changing it using a cheating device, normal save file corruption, a transfer error, or a hacked Pokémon may get corrupted and turned into a bad egg when transfered to Pokémon Battle Revolution. A bad egg is not actually a Pokémon egg, and very well may have never been, but because of the way the 16-bit checksum value works in the Pokémon's data structure, this default message might appear.

The bad egg will have a type ???, may sometimes be holding an item (which normal eggs cannot do), and depending upon the way the data is manipulated, it may be reported to have the Pokérus and/or the small black dot associated with it or it may appear fainted. The owner's ID and OT values are both set to "?????," like those on a normal egg are.

The GameShark code for quick level up in the daycare will result in an invisible bad egg. This bad egg can be switched with the last Pokémon in the party via PC and can be used in battle. Another way to obtain a bad egg is to use a GameShark cheat called "Automatically or Easily Catch Pokémon". If you use a type of Poké Ball in battle you will catch one of the remaining Pokémon (randomly) and the Pokémon the Poké Ball was used on faints. The captured Pokémon will go to the P.C. as a Bad Egg. Its battle image is a black egg, while its summary image is a black circle with a white ? in it. Using this in battle with cause instant white out. The egg can be deleted if it is picked up while holding another Pokémon in the box and set back down.

Its description in-game is "This will take a long time to hatch". If it is forced to hatch, however, ? will come out and the game will freeze.

On Diamond and Pearl, there have been reports of people seeing the Bad Egg while in a double battle. It is believed that the way to see the Double-Battle Bad Egg is by attempting to use the 1-Hit KO cheat for action replay. Getting a Double-Battle Bad Egg requires the user to do the 1-Hit KO cheat correctly (using it before the attack animation begins) to knock out either of the opponent/opponents Pokémon. Then attempt to use it while the animation is going on. However, instead of knocking out the Pokémon, it will act as a normal attack and if that attack kills the foe anyway, multiple things could happen. It could say that the Pokémon fainted, but when the user tries to attack the next turn, after the user has selected an attack, attacking the Pokémon that just fainted is still an option. That Pokémon will then faint again and disappear from the attack menu. Then, after about 1 or 2 turns while attempting the cheat wrong, a Bad Egg will appear in the spot of the fainted Pokémon. It will not show its HP or its level, but it is believed that its HP is 0. Its Sprite is invisible. The Double-Battle Bad Egg has no attacks and holds no items, so instead, it uses held items and moves of the Pokémon before it. When the Bad Egg is knocked out, it will use the cry of the Pokémon before it. But there is also the Copycat Bad Egg that appears only in double battles as well; however, it is much rarer than the Double-Battle Bad Egg. It is triggered the same way as a Double-Battle Bad Egg, but instead of the foe's Pokémon fainting and falling off the screen, the Pokémon's Sprite is still there. When it attacks, its name is now Bad Egg and its HP gauge has disappeared as if it had fainted. It uses the attacks of the Pokémon before it and its cry is still the same. Sometimes, encountering either of these Bad Eggs may freeze the game and it is not recommended that they are captured as they could corrupt or erase saved data. This is not confirmed, however.