I'm guessing a small clump of dust was blocked in the opto, and lodged free during your ball tests etc...

Since it's cleared, maybe give the playfield and opto area a few mild thumps to prove any loose wires that need soldering..

Also, you said you haven't played the game for a while, there's a chance a small spider has found a home in the opto cavity, the spider may have fled the scene while doing your ball tests, i wonder if this fault will be back within 24 hours when spidey comes home?

Now that you've stopped laughing, i see a LOT of these bug-faults in all kinds of gear i fix, especially stuff out in garages where spiders, moths or roaches find a new home.

You might need a bug-repellant strip under your playfield 

MM.