My Turn
Oh gosh, where to start. I've been in the flying biz since high school, lo these many decades back. I've seen lots of dumbth in aviation, but the worst I can think of right now was when my main boss at the time... not me! came to the airport one day, loaded his family into his Cessna 310, taxied out and took off with the electric power towbar still on the nosegear. His first clue was when the gear failed to show an "up" indication on retraction right after takeoff. But that was just a clue, he didn't know what the problem was until he asked the tower to look at his gear as he flew by. They told him what they saw and cleared him to land. Dumb thing number 2 was landing in the grass in case the nose gear collapsed. It didn't and he taxied back to the hangar. Next day, my actual boss and mentor, an experienced A&P and I jacked up the airplane and cycled the gear a few times, checking for problems. All we could find was the pointy fiberglass nose was chipped where the Robotow swung up and tapped it, and the landing light, mounted on the nosegear leg, was broken. Some days later, I was working in the hangar by myself and noticed that the towbar had been left on the nosegear (again) and some wag had wandered in and put one of those long pole, orange bicycle flags on the towbar, visible from the cockpit. It said "Wake up (his name)!" Then the main boss rode in on his motorcycle, told me good morning and asked who put the flag there. I answered honestly that I didn't know. He took the flag off, stuck it on the motorcycle, returning a few minutes later, and put the flag back on the towbar. No idea where he went.
Fair enough, earlier, I'd found an FAA Sarcastic Award plaque in the hangar, awarded to the guy by name "For Excellence in Preflight Inspections and Runup Checks." We already knew he didn't preflight and never did runups. He'd been warned, but still managed to do all that. Plenty of stories about my goofs, but I think this guy has me beat.