Hey it happens. I had a similar experience 20 years ago, I had a 340 duster with a 4spd and no ebrake. I parked it in front of my house and put it in gear since my street had a slight slope to it.

I came out and thought someone stole the car. I looked and saw the car about 6 houses down the street resting against a tree. No damage, no one got hurt. I fixed the ebrake the next day!