No jackets in car seats

I learnt this rule the hard way this past week. I had my two year old wearing a light fleece and nylon jacket. It's just the inner piece of his three in one jacket. I knew better than to put him in a puffy jacket and I thought this one was safe. I tightened him as tight as I could and was driving when he un did the chest clip and slipped his arms out and grabbed the back of the seat.