From my internet wanderings, I observe that people love a punching bag. If they find a person that has erred, they will continue to punch the bag over and over, possibly because it makes them feel better, and particularly if they don't have to see the person's face, get their reaction, and if the person is someone "easy to hate".
Point out the initial flaw, and maybe discuss how to avoid the flaw, and then leave the subject alone. Joining the mob in spitting on someone just harkens back to darker days.