There is a guy who recently started working out at the same time I do who does every single exercise using his entire body and as much momentum as possible. i.e. a bicep curl starts slightly bent forward and ends with his back so arched he is looking at the ceiling. About a week after he joined he brought an older man with him and started training him saying specifically "get your back into it"

A few days later another guy joined with them. Then a woman. Then another woman. He is "training" them all.
My favorite bit was when one of the gals on the assisted pullup machine said "To go higher, you go lower right?" and he said "No, to go lower you go higher".
Not surprisingly his gym ettiquette is also lousy.
I work out as the same time as a physical therapist that I know pretty well. He will correct some form, usually on people he kind of knows and mostly to make it work better.
But not when people are posing as experts. He knows that to correct someone like that is pointless and potentially dangerous. My exercise buddy and I made eye-contact at the "put your back into it" comment and after I whispered "hey look, future patients"