Very terrible.
I've see things like this before. Sometimes a person will try to hit the break but will hit the accelerator instead... another time I saw this happen the person's foot got hung up sideways behind the break so that the accelerator was being pressed and he could not remove his foot.
Once I was driving down the road and when I released the accelerator to slow down the car kept going… and actually started to accelerate. I punched the petal a few times but that did not help. The car kept accelerating. I put the car into neutral and stopped it on side of the road with the engine racing. When I looked down at the petal I saw what was wrong… the floor mat had the corner flipped up so that it was wedged against the petal holding it in place. As soon as I pulled the floor mat back everything went back to normal. Very scary.