RE: What
Early in my flying career, I adjusted the throws on rudder on a Sig Kadet 40 trainer, and didn't visually re-center the rudder. After launch, with moderate wind, the plane started flying in circles and I got disoriented. Plane got lost over the trees and ended up flying into the side of someone's home. I gave up RC for about 15 years for fear that I might end up really hurting someone.
didn't do a full pre-flight on my biplane because talking to another pilot while getting ready to fly. Didn't notice that I had the radio setup for a different plane. Took off with reversed aileron throws, didn't recover.
During radio check, 1) neglected to set Throttle Hold AND 2) didn't fasten the EDF F-16 to anything.... THEN accidentally droped the radio while re-extending the antenna.
Radio went to full throttle, and EDF jet shot across the parking lot into the curb, totally destroying the nose, intake and front landing gear.
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Note that all of these were pre-flight mistakes that could have been easily avoided by some pretty simple steps.
Hopefully. I have learned to be more careful and methodical