Everyone,
Thank you for your help. I've solved the problem.
I have a high speed engraving motor attached to the spindle of my mill. Everything works great and is nice and solid.
While pondering the latest nasty, I looked more closely at the cutter protruding from the spindle chuck. It seemed to be sticking out further from the chuck than I remembered. Sure enough, the cutter had 'pulled' its way partly out of the chuck! Obviously I had not tightened it sufficiently. I am astonished at how forcefully the cutter clawed it's way down, even making inroads into a metal surface underneath!
I think Homer put it best when he said "Doh!"