My Favorite John Candy Movie Scene...

Planes, Trains and Automobiles (1987) - You're Going the Wrong Way!  I think of it so often these days, mostly in context with Israel.

(condescendingly)... "Yeah, yeah, how does he know which way we're going??"

(You can try and warn, but some people are so confident in their own mistakenness, they won't take the warning and will have to take their chances out there and learn from lived experience... if they live through it.)