Racism or chauvinism toward people in the global south.
I can talk with a person who has misconceptions about socialist states due to anti-communist indoctrination. I can't do much to change someone's mind who just views non-Europeans as subhuman.
And the same goes for people who look at the poor as "lesser than" and blame their situation on "their own failures".
Disagreeing on politics, economics or history is one thing. A lot of people are ignorant, uneducated or miseducated. That's just something we have to learn to deal with. But when it enters the territory of dehumanization that's when it crosses a line.