A co-worker in Arizona once had the lock come off the retaining 'hook' on the collective in an AS350 whilst doing a hydraulics check on the hospital pad. With the hydraulics switched off, the collective immediately sprung almost full up, causing the helicopter to leap into the air in a wild spin.
He was able to turn the hydraulics back on and re-gain control only just in time to prevent the aircraft from crashing headfirst into the emergency room. Not that already being in the ER would have helped him any; he would have wiped out the entire staff.