This was to be the front spool end of a reel.
The very last machining operation is to plunge mill 10 larger lightening holes. It is a repetitive operation; plunge, index 30 degrees, plunge again. But at this radius, the part already has two countersunk holes meant for the knob and counterweight screws. So while my mind was in AUTO mode, I wiped out one countersunk hole.
I write this blog more for myself than for any audience. By carefully documenting stupid errors, I might better avoid them in the future.