Probably the whole "pterodactyl" thing. Or calling pterosaurs/marine reptiles "flying/swimming dinosaurs". Really I can't think of anything stupid I've heard people say, but things like those definitely annoy me. Also calling crocodiles "living dinosaurs". In all seriousness, they're not wrong, but crocs aren't as related as birds.
^^How do we know that? I'm not arguing against it, just want to know where that comes from based on fossils.