I'm glad I found this post! I have a fireball classic with random pop bumper and sling shot firing I also have a silverball mania with random pops. I thought the same as everyone else at first switches need adjusting but that's not the issue. Does this mean that switch diodes are not part of the problem. I'll try separating the wires before worrying about switch diodes.

Thank you, that's exactly what I needed!

I found that I already had the switch caps on hand so luckily I didn't have to buy them just for this. Switch caps didn't help, pounding the playfield doesn't trigger them, blocking the contacts doesn't help.
I separated the wiring for each connector from the main bundle and zip tied them individually so its not a tangled mess. After that the random pops and slings went down a lot. Its far from perfect but much better now. It was really bad before now its tolerable.
I also had 3 different mpu's in the machine while working on and testing other things and that made no difference. 2 Bally -35,s, 1 altek MPU.
Would it make any difference if I added caps to the EOS and cabinet switches or shielded wires for the flipper buttons? Its seems like all this is a YMMV situation overall.

Thinking about it, its one pop and one sling both on the right hand side of the machine that are the issue. but after the separation of the wiring the left flipper seems to trigger it more now than the right. I thinking the actualy playfield wiring may be an issue also. I don't know how motivated I am to re wire the playfield on this game.

I'm still going through re reading this whole post again. Lots of things covered all helpful.
I may as well replace switch diodes while I'm at it since there isn't a ton of switches on fireball classic.
Separating the wiring definitely made a big difference and was not as time consuming as replacing switch caps and diodes. I would estimate 50% better so its far from perfect.
It has switched from the right to the left flipper primarily now which seems odd but its still better.

I picked up an Embryon shortly after the Fireball Classic and it has a phantom pop maybe once per game. I can live with it as it is.

Fireball Classic is every couple flips either the right sling or the middle/right pop goes off sometime both seemingly at the same time. Its not always using the flipper buttons thats activating them. The ball will hit the pop then the sling goes on its own. The ball trough kicker also makes the pop bumper go off sometimes.
I also noticed today that occasionally I hear a rollover switch sound when either the pop or sling phantoms.

I'm slowly replacing all the switch caps and diodes so far its not helping aside from fixing the self test switch issues I had early on.
I took the solenoid diver board out of Embryon and put it in Fireball and it was exactly the same, no change.
So i have eliminated the MPU and solenoid driver board.

I'm trying to use logic to solve this but there are too many things going on.

Its driving me batty so I spent this morning separating the playfield wiring. Pulling the coil wires one by one and re grouping them. So far i've got the left side separate but not the right.
It looks like a mess right now, because it is, but i'm going to go back and tidy it all up later.
I played 2 games (10 balls) and the right sling did not ghost pop once that I noticed. It was popping so frequently that 2 games not popping is amazing.
However the middle pop bumper is still ghosting frequently maybe slightly less but hard to tell.
Still more work to be done but its looking slightly promising.

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I'm not THAT motivated to run the whole harness in the dishwasher. I can see the benefit. All I really did was cut all the ties and un thread the tangled mess of coil wiring one wire at a time. It surprises me to see HOW woven the wires are though out the playfield. I think I will take air compressor to it before I button it all up though. I'm wondering if there is any benefit to running the coils in the ultrasonic cleaner?

Its raining right now so I think ill blow things out another day...
I fixed up the wiring a bit more. Lot of wires got shortened and tidied a bit more. I'm going to see if I can get some standoffs like stern uses to keep the coil wiring raised.
Played a few more game. Big difference. Sling shot is not ghosting, if it is at all i'm not noticing it. Now the top AND middle pop bumper are ghosting but its way less frequent now.

I took the ball out of the tilt rack and that didn't help.
I actually separated the flipper button wiring in the cabinet like others have done and that made no difference.

Extra wire:
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Did some more work on Fireball Classic today. Finally found the source of my phantom rollover switch. I followed the wires and found it is daisy chained to a standup target and found a capacitor that I missed when I was replacing them all. Replacing the cap on the standup target fixed the phantom rollover.
I'm learning a lot from the info in this post. Like the switch matrix video vid posted. And double check my work...
Still have phantom pops but it is at least tolerable now. I'll keep poking at it.