What happens when you put atf in a fron diff?

You will start to wear out your ring and pinion gears. Little pieces of metal will start to float around in your differential and those will break off a bigger hunk of gear until one day a tooth comes off and wedges in your spider gears as you are going a corner on a cliff. Your front tire locks up sending your vehicle crashing over the side of the cliff. Good news if your mother in law is borrowing your car. Bad news if you are driving your kids to school. Just drain that stuff out of there and stop putting fluids where they don't belong.