Well, to start, it definitely depends on the railroad. Some do it all the time, others rarely.
Alaska Railroad almost never does it, but on occasion the wrong car gets pulled and sent somewhere it should not be. Most of the time it is caught before it leaves the yard, if the clerks are on top of things, but not always.
When I worked for BNSF, it was a daily occurrence. Typically the way it happened there had more to do with the dispatcher and/or road crews. I worked in eastern Montana, and I can remember several occasions where an eastbound car was scheduled to be set out to the elevator track in Terry, MT. The west facing switch on that track was out of service indefinitely, so it was a move best handled by an eastbound train. Sometimes the crew on the eastbound train carrying that car would be told, by the train dispatcher, to just forget it, because he had trains to move. So then the car goes too far east and ends up in Glendive (usually) or Mandan, ND. Well, a yard crew there gets it, realizes it has to go west to get to its destination, and sticks it on a westbound train. Trouble is, getting it spotted from a westbound train would require runarounds and a lot of extra time, so it goes on to Laurel, MT, where Montana Rail Link would get stuck with the thing. Since they cannot very well deliver it to the customer, they stick it on an eastbound train, to get back to Terry, and the cycle repeats itself. Sometimes an overrun like that would happen once, sometimes a car would get stuck ping-ponging back and forth for weeks and weeks.
Sometimes, inexplicably, you would be switching in the yard and a car would show up on a track that was not on the list. Later when you tried to locate it in the system, it would show sitting in a yard in Amarillo, TX, and you would just wonder how it traveled that far unnoticed. Sometimes the opposite would happen. You would be looking for a car and could not find it. It was not in the place it said on the list, so you check the whole track, but cannot find it. If it is a slow day, you might even walk all the tracks in the yard, and not find it. The computer system and all the paperwork says it is there, but it is not. Then you put it on a "phantom track" in the system, or maybe a "lost car track." These were not real tracks, but it was a place to put cars on the computer, since the system recognizes that all cars are supposed to be on a track somewhere.
Of course on any railroad there is the possibility that the yard crew will mix up numbers. After a long day, CHTT 609149 can look a lot like CHTT 609194, and then someone gets a car they were not expecting, and someone else is missing a car.
