Quoted from pinballaddicted:
If it was my game, I would start looking for a dodgy connection on the bookcase optos. You may have to reflow the solder on the opto boards or electrical connections. Check the opto electrical connections (wiring) are firm and making good contact next.
Hmm, while it could be something like that, I'm skeptical it's a problem with the opto connections, since the bookcase/optos are working perfectly by themselves. The optos register exactly when they should during test and during game, so that makes me think that they're probably fine. Also, I don't see how a loose/bad opto board connection would cause the opto to close/fire when the right flipper button is pressed. But I suppose you never know. I appreciate the advice, and I'll certainly give them another look at some point.
Quoted from GRUMPY:
Replace the diodes on the flipper coil first. Retest. Still bad, check the 12 volt power going the the opto board while flipping. What do you get for a reading?
That crossed my mind, but the thing is the opto phantom-triggers even during test when the coils aren't energized. So it's not even actually the flipper and coil itself that is triggering the opto, but simply the flipper button. As for checking the 12 v power going into the opto board, where would my test point be? I've never actually had to check power going into a board this way. Is there somewhere on the board that I can check, or would I need to pull the connector and figure out which wire was the power line?
Someone else had almost the exact same issue here: https://pinside.com/pinball/forum/topic/100-challenge-help-me-fix-my-taf
It was eventually solved by correcting the orientation of the diode on the switch that was causing phantom results. I do think this TAF problem is solvable, but at this point I think I may need to actually have this game in my workshop area. As when I was called out, I was hoping/expecting to find a misaligned or broken bookcase opto, and this is definitely something more involved. Problem is, the game is practically stuck in the guy's basement due to a narrow and steep stairway.