People who profess that it's okay to let stupid people really hurt themselves. Especially those who cite "gene pool filter!"
That came up after I had to educate somebody that there are seriously major differences between carbon dioxide and carbon monoxide that goes beyond mere pronunciation. This person thought that both were inert smothering gases and that CO exposure was okay in a ventilated space. (Enough to set off the ceiling alarms!) His answer was to open the windows until the alarms stopped.
I felt compelled to elucidate his error and further more I wouldn't leave him alone about it until he found the source. (Roofers blocked the water heater flue.)