I was actually able to be right there when the problem happened this time. The hot end was striking plastic, like the Z had not dropped to the next level.
This is a completely different kind of problem. I wonder why it always shifts the same direction? Maybe that belt is looser or that stepper is just a little weaker...
This is why my exercise program showed no issue, it drops the Z about 5mm and then jams the X/Y directions around. I need to add a Z movement section to the test.
DLC