Funnily enough this happened to me recently :blink: Ultra series arm, three years old with a digibeta 790.
I was filming with a (very short) actress and had the arm boomed all the way down, and just standing there not even moving when there was an almighty BANG and a smell not disimilar to gunpowder.
The spring in the arm section closest to the socket block had snapped and was hanging out of the end!
Thankfully no injuries, just new underwear :(
FYI Tiffen said the cables had crossed.
Marty Newstead