I had a bad egg on platinum and it hatched once into a square that sparkled..

Definately a glitch. A Missingno type glitch where there's no actual Pokemon data, so it can't show you anything because there's no tangible data.

First find out what part of your party the egg was and put that thing in a box and release it I've hatched one before it only froze up when I got exp on it and it looked like any random poke only with diff. Color, I released it when I found out the dangers

Bad eggs are punishment for hacking, you need to get rid of whatever thing spawned from that egg. If you cannot, you may need to restart your game to save it.

It's a missingno and the bad egg messes up your game.make sure to not save your game if it says bad egg or you encounter a question mark.

Oh my! I have a square that sparkles! I meet it differently. I found a square in the tall grass and I accidentally saved the game. Now I am soon going to see if it goes away if I use a cheat that changes the team. Just to let you know: What ever you do, do not put it in the front of your party when accessing the PC or worse, save it after that! If you do that, whenever you look at your party, the game will freeze. I now that because that always happens when I try to get rid of it. At least I never saved it at those times.

Look, there's no way to fix this (don't even think about using a cheat to fix it) now if you have anything egg, quit the game and don't save with it, in my opinion try to leave it at the daycare