How many anti-vaccine / anti-science individuals spout that they know "everything" because they've "done their research" only to find they've learned nothing. Often recanting with regrets and tears if they, or loved ones they actually care about, get very, very sick.
Multiple ones, now, often are recanting and crying in fear when they are in the hospital due to the outbreak, afraid their deliberate ignorance will now be the thing that kills them. :-(
My shitstirring self would ask them some unanswerable questions, like rapid-fire and the instant they say "I don't know!" the response would be, "But you said you knew everything... so you now admit that you don't know everything? You now acknowledge that there's things you don't know?" and then needle them relentlessly about that.
...but I'm an arsehole.