Well, too many to rate biggest bonehead move, so i go chronologically.
First time to take a sailboat out alone. Was 19 years old in a cute little compac 19. Maneuvering out of slip, outboard motor stuck in reverse almost hitting a large mega yacht. Finally realized that I couldn't get motor into neutral cause I had the RPMs too high. I then, in a panic, to avoid said mega yacht, threw motor into forward. Now screaming forward, (RPMs too high), I had the helm hard over to avoid rocky shore ahead. I was pretty sure I'd clear the rocks. I did. But I didn't clear the huge tree leaning out over the water. A thick tree branch wrapped around the mast, curling up and over my spreaders. My sailboat was stuck in a tree, good and stuck. A guy on a research vessel docked nearby saw the spectacle, kindly climbed the tree and untangled me. I limped back to the slip, branches and leaves covering my deck. My first solo voyage distance 100 feet.