Stupid old people. And by that I mean me. Had my car in for service, got a loaner nothing like the one I brought in. Totally zoned out in the parking lot of the grocery store at 5PM. That crowded, dusk chaotic time combined with the fact that I forgot I wasn't driving my car made me temporarily insane. So I'm pulling at the handle of a similar car (different color, probably different make) and am just starting to get angry when Big Hawkeye Fan guy comes up and asks me if there's a problem. "No, just my stupid locks again..." was my answer... He suggested that maybe this car wasn't mine, because it was his...
We've had many listeners unlock and get into cars that looked like theirs, but when they went to start it up, they noticed, I don't know, a giant bag of weed in the center tray. You'd be surprisd how many key types fit multiple models of the same car.
Kids are also famous for doing this at the after-school round-up. Everyone has a Grey SUV...I have kids open my door just in time to hear their mom scream at them...OWEN! over here...