Brontosauruses never existed
I never knew that Brontosaurus was just a misclassification of a previously-named genus until watching an episode of QI the other day. There's an artist's impression of a Brontosaurus in the dinosaur book I've had since i was a kid, I even told Kate about them the other day before having to correct myself and tell her they never existed! She took it in her stride though :)