From Bulbapedia, the community-driven Pokémon encyclopedia.
A Bad Egg in the party
in Generation III
A Bad Egg (Japanese: ダメタマゴ Bad Egg), stylized as Bad EGG in Generation III, is a phenomenon present in the Generation III, Generation IV, and Generation V Pokémon games that results from a corruption of Pokémon data so that the checksum does not match up with the data's calculations. In Generation IV, sometimes Bad Eggs can hatch into the previous Pokémon, or -----.
Bad Eggs are not really Pokémon Eggs, and may possibly never have been, but instead the default message returned by the game if the checksum is off (which happens only if data is altered badly). Bad Eggs rarely hatch and cannot be released, merely taking up space, though there are methods of removing them—it is possible to trade a Bad Egg away into another game, or remove it by cheating.
Like a normal Egg, Bad Eggs appear with a type of ???, as well as have the same Pokémon status screen as an Egg would. Despite this, the similarities end there, as Bad Eggs can be holding items (such as Cherish Balls, which cannot be taken), be placed within types of Poké Balls aside from the standard kind, and have Seals attached—which prevents them from being deposited in the PC. After that, it is only possible to remove the seal from the Bad Egg by depositing it into the Pokémon Day Care.
Some may be reported to have Pokérus or be cured of it, and may be fainted.
A Bad Egg placed in the first slot of the party will, rather than having a normal Egg menu sprite, have a differently colored version of the menu sprite of the Pokémon following it, such as a golden Bulbasaur, a blue Marowak, or a brown Ho-Oh. It is unknown exactly why the sprite retains the last Pokémon's sprite, but the color change is because of the use of palette 0 for this sprite—this is why Pokémon with palette 0 do not change color.
If it is forced to hatch through use of a cheat code, a ? will come out and the game will immediately freeze.
Appearances of Bad Eggs
When using the GameShark code for quick Day Care level-up, an invisible Bad Egg will appear in the party, which can be switched around using the PC and used in battle. Sometimes, the Bad Egg will take form of the silhouette of a Pokémon in the PC Box. However, attempting to view its summary screen will cause a game freeze.
Alternatively, by using codes to capture wild Pokémon instantly, reducing their HP to 0 yet keeping the battle going, it will be sent to the PC instead as a Bad Egg. This Bad Egg can be removed from the game by picking up another Pokémon while it is held, then setting it back down. This Bad Egg appears differently; instead of an Egg in its status box, it is the "unseen Pokémon" image used in the Pokédex and by ??????????. Like ??????????, using it in battle will cause an instant white out (if used without any other Pokémon in the party).
Bad Eggs can also occur if the player attempts to hack a Pokémon which has an illegal moveset, as the checksums will not add up correctly (as the checksum would use the Pokémon's normal moveset at that level as a check).
Index number 495
In addition to forcing an ordinary Pokémon to have an invalid checksum, it is possible to encounter a Bad Egg in the wild by using an Action Replay on Pokémon Diamond, Pearl, and Platinum and setting the wild Pokémon modifier code to 495 (an identifier of Manaphy Egg, although a Manaphy Egg is rendered as a Bad Egg in battle). The game will freeze instantly on capture when the game attempts to show the Pokédex entry. A normal Egg captured this way (using an identifier of 494) will hatch into a random Pokémon, including Pokémon that do not normally hatch from Eggs like legendary Pokémon and evolved Pokémon, or may hatch into ----- or, like its Generation II counterpart, another Egg (which will then go on to hatch into a ----- itself). Due to being incorrectly generated, whatever is hatched may eventually turn into a Bad Egg itself, and then subsequently hatch yet again. The use of an Action Replay with a "1 hit kill" cheat in Double Battles will often result in a Bad Egg appearing. If the player sees a Bad Egg, the game will occasionally freeze. If Transform is used against a Bad Egg, its backsprite will be exactly the same as its front sprite and the only move it knows is Splash.
From catching other Trainers' Pokémon via cheat
If the cheat to catch other Trainers' Pokémon is used, often the Pokémon caught, if following moving after another Pokémon, will either be named the same as the previous Pokémon, Bad Egg, or have a blank name. There are no other side effects.
Hacked data in Pokémon Battle Revolution
In Pokémon Battle Revolution, some Pokémon that are hacked may become Bad Eggs when they are copied from Diamond, Pearl, Platinum, HeartGold, and SoulSilver. 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 a DS game, it will show up as a Bad Egg on Pokémon Battle Revolution if used in a DS battle.
Generation V Action Replay code
In Generation V, if a wild Pokémon modifier code is used to encounter a Pokémon with a National Pokédex number greater than 649, a Bad Egg may appear as a result. The Egg will flee from battle. Catching the Egg will result in the message for a successfully caught Pokémon, but the data of the Egg will be deleted after the battle. The Egg does not have a Pokédex entry or any status information. The player's Pokémon will not earn any experience after the battle. Due to the removal of Seals as a game mechanic, Generation V Bad Eggs cannot have Seals.