A Bad EGG, named Bad Egg in Generation IV and Generation V, is an egg that one can obtain in all Generation III and on Pokémon games. Though the term is often also applied to glitched eggs in general, it is only used in-game to refer to noticeably-corrupt Eggs, which result from the use of cheat devices such as Action Replay, or Poké-GTS.
How to obtain
If one has an Action Replay and goes Wi-Fi in the Generation IV games, Nintendo think that the player used it and then send the player a Bad Egg in the form of a regular egg. It hatches much faster than a regular egg does, so one can tell when it's going to be a Bad Egg when the screen says "Oh?" In some conditions, the egg is unable to hatch.
After the Bad Egg hatches, the game goes completely black after a question mark comes out, and the battery runs dry. The way for the player to get rid of the Bad Egg is for him or her to start a new game.
It is possible to, through the use of a cheat device, catch an egg in the wild or turn a Pokémon the player already owns into an egg. Such eggs don't have any data specifying what they are supposed to hatch into, so the egg simply hatches into more eggs.
Sometimes, it is not possible to battle with a Bad EGG as its back sprite is undefined.
The Generation III games are the only games to actually make use of the term "Bad EGG". The string is used to identify eggs that appear to have been corrupted. The Trainer Memo section of such eggs' summaries invariably states that they are "A peculiar egg obtained in a trade." Such eggs do not actually hatch unless prompted by a cheat device.
In Generation IV, bad eggs are sometimes found by using a cheat to capture the opponent's Pokémon. Bad Eggs also occur when the player uses a cheat device to catch Pokémon in the wild that are not found in the game, like Regice in Pokémon Pearl, or Mesprit in Pokémon SoulSilver. A normal Pokémon in the PC becomes identified as 'Bad Egg'.
In Generation V, many glitches have been fixed that involves a Bad Egg such as that the game no longer freezes or corrupts. However, the Bad Egg cannot be removed if a cheating device is not used. The egg also cannot be used in a battle or be released. It acts like a normal egg that never hatches. If one manages to get one that hatches, the Bad Egg acts like an ordinary egg and freezes the game, a bug still remained unfixed.