Possible to have a flaky switch, possible to have loose wires nearby in an IDC connector.

LTG : )

It's happened to me on occasion, too. I always assumed the ball didn't go through exactly right, because it's so rare, but now that I think about it---that's probably wrong. I'll keep an eye on this thread to see if you (we) get any more answers.

Switch is probably fine. Problem is usually the wireform switch gate becoming misaligned so that it no longer trips the switch actuator.

I had the exact same thing happen on mine. In test the monorail switch worked but would not register during gameplay. A factory reset cured the problem. Thanks.
Tom.

Had the same problem 2 times today. Normally I need to reset the Simpsons 1time every month. But now 2 times on one day .Was trying to trigger the problem but couldn't do it. Switch is working perfectly, I know this because I have this installed http://www.flippp.fr/pisim.php It needs the monorail switch to turn of the flippers and that keeps working perfectly. Anyone found a solution? I would think it's a software problem or IC problem, but would like to know what triggers te problem.

Yes, I know ancient thread.

It's been observed by many now that there is an issue where the upper monorail switch will stop registering during gameplay. Testing the switch (with a ball) in switch mode works fine. Doing a factory reset always fixes it.

Has anyone ever figured out the root cause?

It's only happened twice to me but I hate doing a factory reset and loosing my high scores. That vexing pinball_keefer shows back up on the high scores and with my remedial pinball skills it takes me forever to knock him back off.