Mustang Pro ball stacked in through

Welcome to my Star Trek! It had exactly the same problem. What happens is both the jam opto and the ball opto get registered at the same time. Whoever programmed it never thought of this possibility, so...nothing happens. What I did was loosen the screws that hold the kickout solenoid bracket to the trough. It leaves the bracket a little loose, which prevents balls from getting stuck in that weird position. Give that a try.

It's not the plunger part, but the whole bracket that you move forward. That being said, it looks like your issue is slightly different. Our Mustang has never had that issue. I'm guessing it's a programming oversight as well, though. Does the game give a "pinball missing" tech alert?