Every now and then we see posts about games that have a locked on score reel. There are various possible causes, but one debug path involves suspecting that a scoring switch on the playfield is stuck closed. Unplugging the playfield is often suggested as an isolation technique, and sure enough the score reel no longer is stuck on (or the associated points relay).
So then the requisite suggestion is made about checking all the playfield switches for being gapped too close (contacts touching) and/or back tabs touching. All of this is dutifully examined, and nope, there are no switches stuck closed, all the contacts look correct, all the back tabs are fine.
And yet there's still a stuck switch causing a score reel and/or points relay to lock on...
Here's another possible cause:
The middle blade that's used to stop the switch blade from bouncing back up too high can short against the two blades that are supposed to be open, if it's out of adjustment (Gottlieb actually calls this a 'backing blade'). If just looking at the switch blades, sure enough, the contacts are open, as they should be on this example of a normally open switch pair. But with the backing blade touching both, electrically they are stuck closed.
It's fairly easy for this to happen. The backing blade can be bent upwards when using a blade adjustment tool on the adjacent blade, and not even notice. Sometimes on switches behind rubber rings, the backing blade is more difficult to see, and when adjusting the switch gaps against the ring, a little too much finger pressure pushes the backing blade too far (again, unnoticed). When staring at the switch contacts to get the gap just right, the position of the backing blade can be missed (I know; I've done it myself).
So maybe the next time a seemingly invisible stuck switch is encountered, this might be one other possibility.
[Hat tip to John for the photo!]