I lost it, twice!

I picked up my pupil this afternoon, the car had been parked up a slight incline, and to move off the pupil would need to do a hill start.
She got herself ready, started the engine, got the bite, as she went for the handbrake, my right foot hovered over my dual pedals.

The handbrake went down, the car started to roll back, as the pupil reved the car, and the car continued backwards, my right foot went down to brake the car, slipped off the pedal and the car hit a post behind us.

I thought it would be good for her to practise the hill start, so I moved the car forward to our original position, gave her a quick reminder of the procedure, had my foot over the brake again and told her to have another go.

It happened so fast, but it before I knew it, 'bump' she lost control, my foot went down but missed and we were on that post for the second time. I couldn't believe it! I really don't know why I missed two times, as that has never happened where I have failed to get to the brake. Anyway third time and everything worked out fine and we left the car park without any incidents. That was one of the most embarrassing moments of my being an instructor, as I can't blame the pupil for rolling back and hitting the post, that was entirely my fault!