Some age old "wisdom" dies hard. There was a whole series of those photos with different kinds of powder including black, flake, extruded. I'm still not sure I want to be the one to test it out.
And then there was the one about how microgroove barrels won't shoot cast.
Lastly, I once made an easy $100 from an idiot who would not believe me when I told him I could put out a lit cigarette in a cup full of gasoline. After he paid me, he asked if it was really gasoline. I handed him a book of matches and told him to light one and drop it in and see for himself. Cost him both his eyebrows, some hair, and a change of underwear. Expensive morning.