One of the QFIs I flew with during my RAF rotary course (well over three decades ago) recounted a tale where things got fraught during a rotors running change involving an inexperienced passenger, an Army warrant officer. This was in a Whirlwind 10, turbine engined.
The passenger had been briefed and was delivered to the aircraft, rotors running on dispersal, after the previous passenger got out from the left seat. For those not familiar with the Whirlwind, it was a bit of a climb up into the cockpit, above cabin level (a bit like driving a Luton van from the cab overhang, having climbed in the front window)..
The burly soldier hauled himself up, plonked himself in the left seat and began to strap in, assisted by my QHI, who had to take his left hand off the collective to do so. The aircraft suddenly went light on the skids and began to lift. My QHI diagnosed it as an engine computer runaway up, so he used the disengage lever on the cyclic to go into manual control. The aircraft continued to lift at an increasing rate, as he instinctively tried to grab the collective. He couldn't initially find it because it wasn't where he left it!
By this time the Whirlwind was approaching hangar roof level. The passenger, who wasn't yet on intercom, realised something was wrong and perhaps understandably pulled the lap straps even tighter. This made things worse, the collective went even higher, now with the rotor rpm decaying.
The problem was that the passenger had allowed the left lap strap to go under then around the base of the collective before he pulled it tight. He inadvertently applied almost full collective and strapped it in that position!
The situation was resolved without the aircraft finally coming to grief, but getting the passenger to unstrap wasn't easy; big and tough as he was, he was very frightened! As a government official might say, "lessons were learned!"