Bad luck for you! A bad egg usually appears through hacking or using an action replay or similar devices. A bad egg cannot be used in battle and sometimes multiply, replacing pokemon on your team. They usually don't hatch, but if they do through a cheat, they will freeze the game. Bad eggs will also sometimes have a seal attached to them, disallowing you to put it in a PC box. As some people put it, Nintendo made it as a punishment to hackers and cheaters. That's all I know about bad eggs.