There are a few users who, when you see their name in certain contexts (usually politics) you just know that comment chain going to turn into a train wreck. I'm not going to name names but anyone who's been here long enough could probably guess a few repeat offenders.

But mostly in my experience it's otherwise perfectly normal users who at some point just decide to post something racist or bigoted, advocate violence (again, usually in political threads) or antisemitism or espouse some insane conspiracy theory nonsense. At that point I no longer care about anything else they might have to say.