You all had a very close call and lucky escape there Dan, just shows how we all need to keep a close eye on all our equipment when we walk around.
It could of been far worse if something had happened amongst the crowd at the Show.
Looking at the photo's it made me think about the M-H tapered half shafts on the "U" frame tractors, those wheels get fast and tight and NEVER come off easy.
I remember a story from my dad, one summer in the early 1940's they needed to remove a rear wheel from a 12-20 to replace the oil seal, with their puller they could not get the wheel to move, so the decision was made to put that tractor on the binder, they backed the nut off and locked it with another nut to hold it in place. They cut all the grain on 500 acres and the wheel never gave way and came lose, in the winter it had to have the blacksmith's torch on the hub to get it to move with the puller.
Good luck with your Rumely refurbishment, it will make a change from "the Challenger in buckets" . Hope you do not lose your M-H interest.