[fian]

If your car seat slides back unexpectedly, the most likely problem will be a decreased ability to press down in the accelerator. The seat would generally slide back only under forwards acceleration. Under braking the seat would slide forward giving positive pressure onto the brake pedal.

[GavinMcG]

It also diminishes your ability to brake. And if you do slide back, and the seat then locks in place, that could be serious.

[cjbprime]

You would keep steering the car, while also attempting to unlock the seatbelt. Depending on the car, you could also activate the parking brake. (In my car, it's a button on the center console, and it can be activated at high speed and will provide emergency braking.)

"Serious" is still not the same as "fatal", even if your car hits something. If your plane stalls and falls hundreds of feet, my understanding is that the fatality rate is going to approximate 100%.