Art is 100% correct on this one. Inapropriate user interface with a breaker bar will do it everything. These spinners are just inexpensive plastic...

I am sure we all excerise the most obvious saftey hint, i.e;

NEVER POSITION YOURSELF OR ANYONE ELSE IN IN THE ARC OR PATH OF PROPELLER.

ANOTHER SAFETY CONSIDERATION IS TO ALWAYS WEAR SAFETY GLASSES

I'd say your club members are all incredibly lucky.