Many years ago as an FO on the 727 the captain was flying a night visual approach w me assisting. He called for the first notch of flaps: “Flaps 1”. I had just been thinking that we were a bit high on energy and his call was a bit later than I would have preferred. Somehow I heard “gear down”, repeated back “gear down” as I grabbed the gear handle and lowered the landing gear. Oops. He was not amused.
Lowering the gear with flaps still up was not the normal sequence of configuration events, but was a common tactic for recovering gracefully from being a bit high on energy.
That was more a matter of me doing exactly what I thought I heard based on what I would have done had I been flying right then, not him. This was not me simply grabbing the wrong knob(s) / levers. But wrong knob / lever errors happen too.
The fact the right-seater was an instructor adds some spice to the mix. Swapping seats from flight to flight depending on whether you’re training an FO or a captain really invites muscle memory errors. I’d be curious to learn what other aircraft type(s) the right seater had flown before the ATR or maybe even was still qualified on at the time.