Being that type of guy, I measure the single-layer skirt threads to keep track of the platform alignment. Most of the time, nothing happens, because the M2 has a remarkably stable platform, but some of the objects I’d done in early August showed more than the usual variation and, worryingly, no discernible trend.
Successive sets of thinwall hollow boxes showed the instability:
Adjusting the platform alignment between each of those sets produced no consistent effect, which is most unusual. The X in the bottom set shows where that thinwall box came unstuck from the platform, indicating that the clearance was considerably more than the nominal 0.25 mm layer height.
Peering under platform revealed something else that was quite unusual:
That washer should be flat against the spider mounting plate. My first thought was a burr on the plate, but that didn’t make any sense, as the plate was clean & smooth when I installed the platform; I’d enlarged those holes with a fine file and would have checked for burrs as part of that operation.
Removing the screw nut and extracting the washer revealed the true problem:
It’s a bad washer!
Tossing that one in the trash and installing a good washer put everything in order:
Well, that’s after re-doing the alignment to un-do the previous flailing around, of course.
As nearly as I can tell, that washer sat there without causing any trouble since I installed the hotrod platform. or, more likely, when I repaired a failed screw. In late July I poked the platform to measure how much it moved under pressure, which apparently dislodged the washer and put the burr in play.
That’s how sensitive a 3D printer is to mechanical problems…
The Smell of Molten Projects in the Morning 
that is so cool, and good to know for down the road if i ever do this
It’s just another reason why 3D printers aren’t ready for prime time; eliminating stuff like that requires sealing the printer, closing the supply chain, and preventing users from fiddling with the overall system. That’s trivial with printers that cost upwards of $100 k and difficult with hardware that might find its way into my shop…
Just like on a bicycle: anything out of the ordinary is Bad News!