Resolved: Bizarre Calibration Problem
I had a very strange error after a spaghetti incident where a print had failed. The subsequent print failed during calibration: while positions 1,2,3 checked out ok, position 4 was not detected and the extruder pushed down hard on the plate, generating the error, which stated that the sensor was not detected, "debris on the nozzle?" and that a reset was required. Machine unusable.
I was puzzled, and cleaned off the very minor material left near (but not on) the nozzle). This did not clear the problem.
I used a box cutter knife to fool the machine at position 4 to see if it would proceed. It did, but had problems with position 5 (center) and sometimes 6, but 7,8,9 worked ok.
I finally realized that while the sensor hit the "O" marker on position one dead center, it was way off on the last test, position 9. How could this be? I inspected the y-axis movement, which might be causing this problem. I noticed that the belt would rise on the toothed gear slightly each revolution. Looking closer, I discovered that a small amount of debris from the previous failed print had somehow embedded itself in the notches of the gear, making each revolution slightly larger than designed. This is what caused the position 9 test to be way off (but it must have found some metal by pure chance), and also caused the center positions, 4,5,6, to be misaligned.
I removed the offending material and guess what? The printer worked perfectly once again.
Moral of the story: if you have a bad print incident with material all over the place, check the belts and gears on the motion system in addition to the extruder and hot end.