Bad EGG

A Bad EGG is a corrupted Egg that a player can obtain in Generation III games. Though the term is often also applied to glitchy Eggs in general, it is only used in-game to refer to noticeably-corrupt Eggs, which result from the use of cheat devices.

In Generation II
It is possible to, through the use of cheat devices, catch an Egg in the wild or simply turn an already-owned Pokemon into an Egg. Such Eggs don't have any data specifying what they are supposed to hatch into, so they will simply hatch into more Eggs.

In some cases, it is actually possible to battle with these Eggs; however, attempting to do so will freeze the game, as their back sprite is undefined.

In Generation III
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 to with a cheat device.

Bad EGGs have a reputation for being highly unstable and corrupting save files. They are thought to replace random Pokemon in the PC; ruin Hall of Fame data; freeze the game; and erase save files.

In Generation IV
Bad EGGs can be obtained in Pokemon Diamond and Pearl using any number of faulty cheat codes with a "game-enhancing device".(Usually an Action Replay) Depending on the specific codes used, a glitched Egg may be identified as having been obtained on January 0, 2000 in a Mystery Zone.

If a noticeably-illegitimate Pokemon is created using "Pokesav" and transferred to Pokemon Battle Revolution, it may become a Bad EGG.

One means of getting a Bad EGG is to use a cheat that instantly places an Egg in the Daycare without actually having any Pokemon stored there. Upon receiving the Egg, a player will find that it has become invisible. Withdrawing a Pokemon from the PC, however, will lead to severe game glitches. Using a glitch to determine what the Egg will hatch into will result in severe graphical abnormalities. The Egg will, if prompted to using cheats, hatch into a Pokemon identified as "--"; sending this Pokemon into battle is known to have negative effects.

Bad EGG can also be obtained in HeartGold and SoulSilver through the use of multiple "999 of one item" Action Replay cheats at the same time. It will turn your last Pokémon into a Bad EGG. The Bad EGG will say that it was obtained January 0, 2000, in the Mystery Zone.