Well there I was just milling away with an 1/8" ball endmill in plastic, taking a .080 cut and all of a sudden it stalled and started making a very loud metallic metal to metal noise from back by the y axis drive motor. E-STOP don't fail me know!
After I reset, I tried to jog the y axis and all I got was the same loud sound. I check the drive and it moved by hand when I turned the lead screw. No motor brake....crap!
I pulled the motor out and tore it down to find the shaft had sheared inside the bell housing. Thank goodness I had that spare motor sitting there waiting for that (one of these days ) 4th axis. A quick swap and I was back in business about an hour later.
Man that sucks. On two separate occasions I've had a leadnut come apart on me and the setscrew on a motor coupler vibrate out - both times the part was ruined. They were simple fixes, but it's frustrating to waste time and materials like that. Luckily you were nearby to stop the machine before it broke your bit or ran it into a metal clamp and damaged your router.