They tried to argue that all math was useless bullshit because it couldn't deal with infinity.
I tried to show different ways math has been useful for me personally, and for humanity generally. None of that mattered to them. I tried to explain how math can actually work with infinity. They insisted I was lying.
A part of me thought they were just trying to troll me, but after seeing and interacting with them multiple times afterwards, I'm pretty sure those were their genuine beliefs. They were also a moon landing denier, but after the whole math discussion I didn't touch that topic with a ten foot pole.