Not quite the same thing (adult seats vs car seats, and it's possible they were just faulty from the factory rather than expired), but the last time I was rear-ended the seats snapped behind me. Quite the experience really -- to stop you from going forward you have seat belts, air bags, and all kinds of fancy engineering. Inertia is a cruel mistress when the car moves forward out from under you though, and the only things ensuring your forward motion are a couple of broken seats (thankfully there weren't any passengers or children in car seats for me to smash into) and the 2-ton metal projectile poking through the ass end of your car.