Very nicely illustrated! Thanks, xsvtoys!

To try and answer your question above, as you mentioned, the blue drive-gear shaft with the threads will need to "drop out" to help the motor stop immediately. If this drive gear actually stuck in the drive position inside the motor, it would drift to a stop, and all the motor switches might not be in their normally functioning position. There's probably another braking device on this motor set-up, but it's not strong enough to stop the motor cams where they need to be.

Another symptom as a result of the problem you've found would have the motor drifting to a stop, but begin another revolution again as the run switch accidently makes from the motor cam drifting too far. The symptom you would notice then, is that the motor never stops turning. I've seen this a few times on Gottlieb Games, (although the motor set-up looks quite a bit different, underneath, you'll find the same type of drive gear used).