Well you don't just take the baby out the seat and chuck it in the back of the car through a cracked in the door. An adult can squeeze past, car seats and not so easy to squeeze in
Really though, this can be prevented by just leaving the car seat in the car where it belongs and putting the baby in a sling/pushchair/pram/infant seat of a trolley. Its better for the baby to not be in the car seat, better for your back, and saves you from not being able to get the seat back into the car.