My first car, a 1966 Buick Skylark, had the same problem. When I was 18 (legal drinking age in Virginia at the time), I was rushing into a store to beat the midnight deadline to buy beer and forgot to put the car in park. I just grabbed the keys and ran for the store.
When I came out my car was out in the street and there were about eight Spanish guys pushing it down the road. My first thought was that they were stealing it. I ran up to them and started pointing at my car and yelling at them. They started yelling back and one of them pointed to the spot where I had parked it, then drew a line with his finger out into the street where we all were standing. It was then it dawned on me that these guys had just done me a huge favor.
Fortunately, I had bought a lot of beer (I was heading back to a party), so I popped a case open and started passing them around. You'd be surprised to know how much beer eight Spanish guys can drink in a parking lot at midnight when the beer is free. After another round or two of "Gracias" on my part I tossed what was left of the beer in the back seat and went on to the party, where I was promptly accused of drinking it all myself and making the whole story up.
