Quoted from nthack:
It does affect the game significantly. There is confusion throughout with the machine getting in a pickle and not knowing where the balls are. Sometimes two balls in the right pocket. Sometimes repeatedly upping the ramp and shooting balls in to the pocket, then the gate opening and dropping the ball out. Sometimes no ball in either gate when the mist starts.
Well, that info is more useful than "I get a T15 error".
This is probably not a long range opto symptom, but I'm guessing. Long range opto issues causes the ball to drop mid travel for no apparent reason.
Sometimes two balls in the right pocket:
optos in the right ball pocket are not working well, either dirty, misaligned, a broken wire somewhere, not a matched opto set, bad opto board, loose connector somewhere, opto soldered in backwards,etc.
Sometimes no ball in either gate when the mist starts:
again, the opto's in either ball pocket seem to not be working well. See assessment above.
1. Clean the all optos with a q-tip and some non-ammonia window cleaner, do it 2 or 3 times (wet q-tip, dry q-tip... repeat...).
2. Did you replace any of your opto's or just one side of the opto chain?
You should get opto's that are supposed to work together, the emitter and the receiver. Not sure which one is the issue, but some combo's of Emitters & Receivers (by different manufacturers) don't work together well.
3. Replace (or exchange with a known good set...but that's a lot of work and can cause more damage, better to just replace with new) optos in the right ball pocket to see if that helps, those are the ones that control ball replacement.