I've had my share trying to convince someone that a perpetual motion system they described did not conserve energy or momentum. They refused to believe my assertion that momentum and energy were conserved quantities.
It was a long forum discussion, and well over a dozen engineers, mathematicians, and physicists kept saying the same thing. Some wrong long (>500 word) posts. We were all ignored. This person was invested in the idea that they had an insight that centuries' worth of work had overlooked.