I was parked pointed forward in a parking lot at the top of a fairly steep, but rather short, decline. The car was fairly level, but I might have been just a little over the apex of the decline. I started the car and released the parking brake (the car was still in Park). For some reason, the car slowly started rolling down hill. I pressed on the brake as hard as I could but the car wouldn't stop. I don't know if the car had stalled, which would have rendered the brakes inoperative, but all I know is I couldn't stop until I ran through a fence at a baseball field and ended up in rightfield. I made sure I didn't touch the gearshift at all after stopping, and another individual confirmed that it was still in Park after they got into the car to move it. My biggest concern is that I applied the brakes as hard as I could and it still would not stop the car.