What's the weirdest argument you've gotten into with someone?

https://lemmy.eco.br/post/21858886

My colleague doesn’t believe in Einstein and his discoveries. I said that things like spacetime and the relativity of time were in fact proved but he insisted they were all lies. He doesn’t even believe that the speed of light is the fastest thing ever because it’ll be proved only when we’ll go that fast in first person loool
It’s like an expansion pack for flat earthers.