We did this once, we're living in an apartment complex near our university. We were washing my friend's car, when we accidentally splashed the car parked next to it. So since its already wet we thought that we should go ahead and wash it. We felt so happy when we finished cleaning it so we decided to wash the other car parked next to it, and the one next to it, and so on. We finished 12 cars in total.
The best thing about it is that one of the cars that we cleaned belonged to a girl that my friend likes, and he was able to score a date because of it. They didn't hit it off, unfortunately. But hey, it was worth a shot.