John DavidRichard Owen, in 1842, defined dinosauria as the last common ancestor of Iguanodon and Megalosaurus and all the descendents of that ancestor. However, for the past several years, we have found evidence that ornithischians (Iguanodon) evolved from saurischians (Megalosaurus) after the Sauropods split off. If the recent evidence (e.g., Baron et al. 2017) is correct, then by Owen's original definition such classic 'dinosaurs' as diplodocus and apatosaurus are not true dinosaurs, but flamingos are.
I think we're getting used to the latter (that birds are dinosaurs), but I don't know if we will ever accept the former (that sauropods aren't dinosaurs).
This is true, by the way, though there's some context and nuance I had to leave out for space. We don't use Owen's definition much anybody, but the common modern definition of dinosaurs as the last common ancestor of Triceratops and modern birds and all its descendents also implies the same thing.