Along the dinosaur route, I remember when the Brontosaurus existed. Then they were suddenly like, "Nope, Brontosaurus was never actually a dinosaur." And I was sad.
Pluto is the obvious one. Still kind of annoyed about that.
I actually just learned about the Southern Ocean on the TrekBBS a few months ago. I had never heard of it before. I also learned that different countries have different ideas of how many continents there are; I always assumed that was a universally agreed upon concept.