There are many things that take me out of fic but don't really upset me. If I come across a fic that has no capitalisation I just back out.
My greatest grief is in very particular fandoms- those that claim they are going to explore something realistically when they actually mean grimdark. This usually involves works with magical beasts and starting to treat them like various animals from our world. Expect it's clear they didn't do any research of animal behaviour. That their reference pool is "insert eco-terrorist propaganda from your country".
Having friends, family and myself having years of experiences working against those kinds of people, I just... I cannot handle that well.