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. In these games, Bad EGG is called simply EGG.
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. In these games, Bad EGG is called simply EGG.