Believe it or not, this actually happened to me. In my university days I had a lovely green '79 Mitsubishi Lancer GL. One day, I hopped in, started the car up, reversed out and had the feeling something wasn't right. I noticed the faint smell of cigarette smoke (I am not a smoker) and wondered if someone had been in my car. It was only when I opened the glove box, only to recognise nothing in it; that I had indeed taken someone else's car. I was mortified, so discreetly reversed back, and parked the car, locked it and sheepishly found my car parked a few bays down.
A few weeks later I saw the same car, and for shits and giggles decided to park next to it. As it was exam time I found a nice observation spot in one of the libraries and watched to see what would happen. Sure enough, a parade of people doing double takes, a bemused security guard and yes, the owner of the other car backing my car out of the car park and driving about twenty feet down the road before realising their mistake. The real kicker was that our licence plates were almost the same, the only difference was the last two digits were reversed.