The printed bracket for the M2’s Z axis home switch doesn’t get a good grip on the oiled steel rod, so it can slide around just a little bit when nudged. That doesn’t happen often, but when it does, all your careful alignment Goes Away.
A single wrap of silicone tape solves that problem:
While I was in there, I replaced the socket-head cap screw I’d been using with a longer hex bolt and swapped the nylock nut for a plain nut that’s easier to adjust. I should file the raised markings off the top of the bolt head so it presents a smooth surface to the switch.
That was easy…