Some of the 150/2s I've flown have had the fuel wired open, others you needed a wrench to close it. Seems some schools/operators just don't like having it fiddled with.
I think a pax pulling the mixture or shutting the throttle is an entirely realistic scenario, some folk just can't help but fiddle. Wasn't a Rockwell twin lost many years ago due to pax interference?
Oh, and wasn't a Tristar lost in the Everglades when they were trying to change the bulb in the undercarriage warning and knocked off the autopilot without noticing?
TOO