The mini playfield's up and down motion is controlled by a motor that CAN become stripped in the gears if it ever "goes sideways." This happened on my unit when a screw fell out of the metal ball-catch when the playfield was up all the way and a ball went in. The playfield started going down and stripped the gears. It required a new motor.
If the opto sensor on the flywheel goes out (either the opto light or the receiver), the mini playfield will not know where to stop, resulting in errors.
If you remove the mini playfield and later put it back in, make certain the cables in the back are not crimped as they can impede the up/down motion of the mini playfield.
The only way to see if the opto sensor is working (without an expensive IR detection card) is through selfie mode on a cell phone. Get the camera positioned just right to detect the IR light (invisible to the naked eye). If you cannot see it, it is not functioning. It is either burned out or unsoldered.
BEFORE REMOVING YOUR MINI PLAYFIELD, READ THIS DOCUMENT: http://www.pinballsupernova.com/images/TheDoctorWhoMiniPlayfieldv1.2.pdf
PRINT IT AND PUT IT IN YOUR PINBALL MACHINE. You will refer to it often if you ever have to work on the mini playfield.