Not a complaint... but a repair story.
I picked up a N scale Kato GP50 at a show today, and ran it around a bit on the layout. Noticing it ran rather fast at low voltages and wanting to confirm there was no decoder inside, I opened the case to take a look. There was a pretty nice drive mechanism, with a circuit board on top for the lights and transfering power.
After putting it on the track to watch how it ran, the locomotive wouldn't move. A couple nudges, and still no movement. Since it's not a bad section of track, I grab the multimeter and check for shorts. Yep, shorted.
So, I start taking the model apart to find the short... I take a screw out on top and the truck falls off, taking a bearing with it. In my attempt to isolate the circuit board, I remove the four screws holding everything together. At this point, my locomotive is now in kit form.
In this form, I've got an idea of how power is transfered to the motor. On the top of the motor, there's a brass conductor that connects to the circuit board. The tricky part is the how the other side gets its power. There's a silver (aluminium?) "box" that attaches to the circuit board, and transfers its power to a brass terminal at the bottom of the motor.
This box passes extremely close to the brass conductor at the top of the motor, and I suspect when I took the shell off to get a look inside the brass conductor contacted the box and caused the short.
This is actually the second time a model's turned in to a kit for me, the first one was a building that had the glue fail due to the high humidity.