The parking mechanism in the transmission failed. The car was parked in my garage . The lever was in the "Park" position when the car drove backwards out of my garage and crashed into the fence. It continued driving into the street and into the neighbors yard. Two people were out walking and saw it happen. I know I did not leave my car in neutral or reverse when I parked it in my garage. When I tested this later sitting in my car, in my garage, the car immediately began moving backwards instantly, while i was still at the wheel. Had the car been in neutral or reverse at the time this incident occurred, I would have known it was moving immediately and shifted it into park right away. When this incident happened I put the car into park, and I got out of my car , I got things from my trunk and went inside the house. Then I went back to the car to get more things. This is when I witnessed it driving out of my garage backwards. I was standing on my back step. Then I ran across the street to my car and got in the drivers seat to attempt driving it back to my garage. I had to yank it out of park and then shove it back into park to get it to start correctly. Then I was able to get it running again and drove it back across the street to my house. thank God no one including myself got hurt. The possibility of this happening shook me up for days. I reported this to Hyundai and waited 3 months for them to make a determination, they said they cant prove it , that it is my word