I had to explain to a coworker that Hindus and Muslims belong to different religions (I am neither, I just happen to pay attention to other people's cultures and their preferences and beliefs from a personal interest standpoint).
Coworker was making an offensive joke about a Hindu coworker and certain things extremists from other religions can get up to. I pointed out to her that he was Hindu, not Muslim and it was an ENTIRELY DIFFERENT RELIGION. She had no idea. Sigh. (I didn't bother to get into the whole offensiveness of her joke in the first place, it would have gone right over her head about why that was wrong as well).
Had to explain to another coworker that offering the Hindu coworker halaal options for an office catering thing was kinda pointless, since he didn't care if it was halaal, he just didn't want to eat beef.