I'm one of those neurotics who double and triple checks that she's got her keys, tickets, ID, etc., so I rarely lose things. I do have a funny "lost car" story, though. When we (my siblings and I) were teenagers, our family ended up with five cars and a truck parked in a long, narrow driveway. One afternoon I ran to the nearby mall for something, and took my brother's car, since it was parked behind mine, and I was too lazy to switch cars around. I did my shopping, and drove home. An hour or so later I got a frantic call from my brother. He was at the police station, reporting my car stolen from the mall parking lot, and needed the license plate and registration numbers. Since I'd taken his car, he'd taken mine. I looked out the dining room window, and saw my car sitting in the drive. All I could think of to say was, "You didn't happen to see your car in the lot, did you?"
Another time I managed to "steal" a car. We had a white VW Golf. I came out of the supermarket, unlocked the car, loaded my groceries, and drove about two blocks before I realized that I wasn't in our car. I drove back to the supermarket, only to find a very irate man standing next to our car. We had the same year, model, and color, and our keys were interchangeable! After that, I put several bumper stickers on the car, and always checked the license plate.