Another logical fallacy I've seen is kind of the contrapositive (if I remember ethics class correctly): If people hear an anecdote of a problem affecting someone in a group, the problem exists throughout the entire group.

E.g. the other day Reddit had a story about an atheist who dropped out of West Point because he felt there was too much religion, which somehow mutated on some of the subreddits to how all atheists in all of the military branches are continuously being oppressed (which is far and away untrue).

Even more astoundingly, even people who've run headlong into this attitude from others still don't realise when they're doing it themselves. For instance, a lot of feminists have trouble grasping that racism does actually exist, anti-racism activists can't understand why sexism is a big deal, etc...