Anything in the coding of the game isn’t going to hurt your game. Having anything you can’t properly obtain in game or through trading could possibly get you banned from online activity, but it can’t break your game if it is programmed into the code of the game.I know back before ORAS was released people were hacking the game and found megastones for the lati and I believe diancie. I also know people would put unreleased gems into their games (I battled someone and couldn't upload the battle vid later and they told me they forgot to remove their ground gem from their pokemon).
I'm wondering, are these things that can break your game (whether you have the cheated items in your game or in my case having battled someone who uses them )? Also could someone explain about the bad egg to me? I've heard of it but I really don't understand what it is.
Example: you receive through wonder trade, a Mew with attacks and an ability it can’t have without cheating. There’s no way that Mew can break your game because the ability and attacks exist in the game even if Mew shouldn’t have them. However, if you try to use that Mew in an online battle, it likely won’t be allowed and you risk getting banned for it.
I don’t really know much about bad eggs in the later gens. I think it’s pretty difficult to get one, but if you do, you should probably delete your save and start over. You can save your Pokémon in bank before deleting your save, just don’t try to deposit the bad egg