I got this occasionally too. My optos all seemed to check out but I cleaned them anyway and it has seemed to solve the problem.

Sorry I've got nothing else. I guess I was lucky and didn't have to dig very deep into it. I think it's a common problem tho.

Should not happen, are they releasing from the castle lock? If so you may need to clean or check the parts for the release plunger possibly.

Mine has never done that in 2 years+

If they are releasing from the through, you probably have flaky switches in there...

It should release one ball from the castle lock for bats, but not for rats. If you cleaned the optos in the castle lock, maybe the release plunger is a little sluggish? If the game sees the right changes at the castle optos, it knows to stop releasing balls. Are multiple balls ending up in the shooter lane, or is it pausing to release another?

I just did a little experimenting on my machine. If I stick my finger into the castle while there is a ball locked, it will release a ball. If I left my finger in there, the lock would keep releasing over and over, but the release plunger never has the little post lowered for more than a second at a time.

+1 on inspecting/cleaning the castle release mechanism. probably dirty and not releasing correctly

Yeah, my BSD did the same thing when I first got it. I adjusted it so that the releasing rod won't touch the PF and that fixed it. The contact with the PF was just enough to slow the rod popping back up down. Fix that and you should be good to go.

Yes you may want to rebuild inspect that area closely - I also had a similar problem that seemed to resolve itself - probably when I was shopping it out I may have adjusted it it slightly and totally forgot about it. But it has to be fast and smooth to catch that next ball coming.