From Bulbapedia, the community-driven Pokémon encyclopedia.
A bad egg in the party
. Note how it is capitalized as "Bad EGG."
A bad egg (Japanese: ダメタマゴ no-good egg) is a phenomenon present in the Generation III and Generation IV Pokémon games that results from a corruption of Pokémon data so that the checksum does not match up with the data's calculations.
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 will never hatch and cannot be released, merely taking up space, though there are methods of removing them.
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 (which cannot be taken) and be placed within types of Poké Ball aside from the standard kind. 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 why this happens, as even Template:Shiny2 Pokémon have a normal-colored menu sprite, however it may have to do with the game using an egg sprite's palette for the first sprite information it encounters (as the bad egg's is blank).
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 Daycare level-up, an invisible bad egg will appear in the party, which can be switched around using the PC and used in battle.
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).
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 (normal eggs appear for 494). The game will freeze instantly on capture when the game attempts to show the Pokédex entry. A normal egg captured in this way 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 DPBox or, like its Generation II counterpart, another egg (which will then go on to hatch into a DPBox itself). Due to being incorrectly generated, whatever is hatched may eventually turn into a bad egg itself, and then subsequently hatch yet again.
The same issues with regards to capturing Trainers' Pokémon and double battles remain in play in Generation IV, with bad eggs popping up in the same situations.
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, Pearl, and Platinum. 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. Use of a 1 hit kills Action Replay code in a double battle can result in one of the fainted Pokémon becoming a Bad Egg. The egg doesn't always freeze the game, but the risk is high.
If the player has a hacked Pokémon, although it won't appear to be a bad egg on their DS game, it will show up as a bad egg on Pokémon Battle Revolution if used in a DS battle.
- Bad eggs in Generation IV that result from cheating while online often are holding Cherish Balls, which of course the player cannot take from them.
- In addition, Generation IV bad eggs usually have seals attached, which prevents them from being deposited in the PC. It is thus possible for all party slots to be taken up by bad eggs. If this occurs, battles cannot be fought at all.