From Bulbapedia, the community-driven Pokémon encyclopedia.
A bad egg in the party
. Note how it is capitalized as "Bad EGG."
Bad eggs (Japanese: ダメタマゴ bad egg) 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 becoming corrupted. 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. If the Bad EGG is placed in the first slot of the player's team, and the player views his/her party using the PC, the Bad EGG's icon will appear as the next Pokémon's icon, but with different colors, such as a golden colored Kyogre, a blue colored Marowak, and a brown colored Ho-Oh. It is unknown why this change is made, because even with regular shiny Pokémon, their icons appear the same as it would with a regular colored Pokémon. Its description in-game is "This will take some time to hatch". If it is forced to hatch, however, ? will come out and the game will freeze.
Why bad eggs appear
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 will 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. The bad egg can also be encountered if tools such as PokemonMaker are used to give Pokemon illegal movesets, etc.
It is also possible to encounter a Bad Egg in the wild with the use of an Action Replay, with the 'Press select to encounter a particular Pokémon' code on, and the wild Pokémon having an index number of 495. Upon capture, the game will freeze when it tries to show the Pokédex entry. A normal egg can be made to appear using the above method and an index number of 494. The normal egg will hatch into either a random Pokémon (even a Legendary), DPBox, or another egg (which will always hatch into a DPBox).
Another code that has the possibility of triggering a Bad Egg is by using the "Capture other Trainer's Pokémon" code. Specific Pokémon caught using this method may turn into Bad Eggs. Some remain as they are, but receive the name "Bad Egg" when the game asks the player if they want to nickname it: (Do you want to give a nickname to the caught Bad Egg?). These may eventually turn themselves into DPBox, and then possibly into Bad Eggs. Also, this method may also cause other trainers to send Bad Eggs out to battle. If these are captured, they may freeze the game. If not, these Bad Eggs will hatch into either a DPBox, another Bad Egg, a regular Egg (Which will always hatch into a DPBox anyway) or, less commonly, a completely random Pokémon (whatever hatches will still be referred to as a Bad Egg, however). Whatever hatches may turn back into a Bad Egg randomly, which will, after a long time, hatch again.
Double-battle bad egg
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 to knock out either of the opponent/opponents' Pokémon. But, as the Action Replay code for an instant faint does not work correctly in a double battle, the second Pokémon will take damage regularly. If the regular-acting attack used knocks the foe out 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. 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.
Hacked data in Pokémon Battle Revolution
In Pokémon Battle Revolution, some Pokémon that are hacked may become bad eggs when they are copied from Pokémon Diamond and Pearl. A common example of a bad egg replacing a Pokémon, is when that Pokémon has more than 510 Effort Values. Bad eggs are unable to battle in Pokémon Battle Revolution.
If the player has a hacked Pokémon, although it won't appear to be a bad egg on their Pokémon Diamond or Pearl game, it will show up as a bad egg on Pokémon Battle Revolution if used in a DS battle.
- Bad eggs in Diamond and Pearl that result from cheating while online oftentimes are holding Cherish Balls, which of course the player cannot withdraw.
- In addition, D/P Bad eggs usually have Seals attached, which prevents them from being deposited in the PC. It is possible for all party slots to be taken up by Bad eggs. If this occurs, battles cannot be fought at all.