The breeding of two hacked Pokemon was said to create a 'bad egg.' Basically, all the way up too BW and perhaps XY, breeding a hacked Pokemon caused this strange egg named Bad Egg. To skip the supposed details, when this egg hatched, the game would crash or break permanently. So these hacked eggs are warned not to hatched, however, often you could not actually deposit or be rid of them, so you had no choice. so instead, you were warned to not breed them at all.