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…
The Smell of Molten Projects in the Morning 
Inverting the screw & using wing nut as lock nut accomplishes:
* Far more convenient adjustment … no tool required.
* More uniform contact with switch leaf.
But, if not selecting proper screw length and/or head, interference with Z motor at bottom of travel.
Ah, would that I also had a selection of metric wing nuts… [grin]
One advantage of hex head fasteners: wrenches have a long-enough lever arm to control what’s going on and make fractional turns quite precise. The M2 remains stable enough that, after I set the bolt by measuring a few actual skirt extrusions, it doesn’t need much attention. I like that!