With my setup now working, I started printing useful things, such as endstops for the wires on Frida's bag and cooling rack feet. I printed several pieces without trouble, but then one of the endstops for some reason didn't raise the head while printing and started to crash into the print. Good thing I was nearby to notice, rather than in the other end of the house.
After resetting the Z axis and oiling it, I tried another print - and the nozzle crashed into the side of the bed! After careful calibration, I found the Z height was now 202.8 rather than the 204.5 I had it set to. Where does one lose 1.7 mm of height in the middle of a print? My suspicion is that the hotend got a bit slanted/pulled when hitting various parts, so now it's at a different height. Sounds dodgy. If it became slanted, it would have more height, not less. If it got pulled out of the mount, it could have more height, but it's pretty damn tight on the mount.
It would be nice to have something that notices when the motors skip, either due to lose connectors or due to hitting something. But then that'd need to be calibrated, too, and could have errors, too.
It starts out slightly high, now. And I did notice the failed print was loose in one corner, maybe the problem was that it buckled upwards and that part hit the head. The UHU stic seems to have two modes, either it's sticking so much I have to use a wrench to twist off the print, or else it doesn't stick at all. Quite possibly sticks better at low temperatures. I should get some hair spray.
Prints nicely again with the adjusted height.
I'm also having small holes being too small, up to 1 mm too small for 2mm holes. Apparently, slic3r is still not good at compensating for what happens in a tight interior space. I see from +nop head 's explanation here that it's not a trivial problem at all, but some more tests by +Giles Bathgate indicate that it can be correctly accounted for - and he uses slic3r, too, so I should be able to reproduce it. I will print that when I have an hour to spare for tests, and once I'm satisfied that my bed can handle the size of that thing!
Every so often, there's a little popping noise, or perhaps it's a sticky noise, I just noticed it happening when going from printing a surface to printing around holes. Not sure what that is, but it's probably a sign of some surprising failure mode sneaking up on me.
The bed fan seems to turn on or off fairly randomly. It works when forced on, but in the current print turned off after being on for a while. Then turned back on. Seems some parts are cooling-free.