Earlier in this thread I asked half-jokingly how many other things could go wrong. Add this one to the list: the material pulled free of a clamp and began to scoot with the spindle. Slap the red panic button. Try to raise Z 2 inches. No response. No response in any direction, even after stop-start the controller and reboot the computer. Tried it in UGS and saw an alert about the z limit. So I sent the command $20=0 to turn them off, and it let me raise the spindle.
It was working on a piece which is a round box lid, cut out and held with tabs. The bit was down at the bottom, making the last cut. I don't know what would make it think it was past the z limit. Maybe it has to do with stopping it with the red button as I did.
But here's the good news: this was the second g-code file. It made it all the way through the one before it with no trouble, and it wasn't the random stopping error that blew this attempt. So hooray for that much.
At the rate I'm using up material in failed attempts, I'm going to need to make a trip to Home Depot soon.