I was told this one by my RAF QHI, true story as it happened to him in the Whirlwind (single turbine).
He was to fly an army WO pax who had been briefed how to strap in rotors running. The WO climbed up the left side, sat down and began strapping in, with the QHI helping him using his left hand to pass the harness straps. Suddenly, the aircraft launched itself vertically upwards. The QHI, assuming the aircraft had a fuel computer runaway up, immediately went computer out (pulled in the big "bike brake lever" on the cyclic) and concentrated on controlling the aircraft which was now rapidly (well, as rapidly as a Whirly could do) approaching level with the hangar roof.
Having now put himself in a manual throttle situation, which made no difference, he realised that the collective was way high and tried to lower it, without success, it was stuck fast.
As he transitioned over the hangar roof (!) it became apparent that the WO had somehow put his left hand leg strap UNDER the collective and pulled it up tight, jamming it in a high position! Unfortunately he was not yet on intercom and not surprisingly, was petrified! The QHI had to yell at him and finally convinced him to unstrap, which was no mean feat considering the pax thought he was going to die in the next few seconds!
Yehaa! They got it down in one piece with the only damage to self respect and pride....
Stupidest thing I ever did was to swap hands on the controls in a Gazelle when I was flying as QHI from the LHS. I was asked to fly for a few seconds by my student as he had got his maps in the wrong order as we approached the departure point. He had the friction far too tight for my liking (friction situated on the right collective) so I stupidly tried to hover with my left hand whilst I wound it off with my right.
Well I shan't ever do that again!! The next few seconds went very quickly and my life flashed before my eyes. If anyone wants more detail, I can say why!!
