In 1874, Othniel Charles Marsh discovered a huge sauropod dinosaur which he declared as a new species. It's name was Brontosaurus. The discovery came during the "bone wars", a period of intense fossil hunting marked by a heated rivalry between Marsh and Edward Drinker Cope. Cope, among others, was not convinced that Bronotosaurus was a new species, but was actually Apatosaurus. Marsh died in 1899 having discovered around 80 new species of dinosaur, far more species than Cope had unearthed.
However, in 1903, scientists announced that Brontosaurus was not a new species at all. It seems that this news did not reach the general public, nor did it reach the U.S. Post office who issued four dinosaur stamps, including one for Brontosaurus. Even museums continued to use the label Brontosaurus on their exhibits. Many saw this as promoting scientific illiteracy, and the media enjoyed the scientific scandal. It wasn't until 1974, however, that the name of Brontosaurus was formally removed from paleontology. It is unclear whether Marsh made a deliberate mistake due to his intense rivalry with Cope, or whether it was an accident.
Yet to this day, people still use the word Brontosaurus despite the fact that it is scientifically obsolete. But does it really matter?
Elmer Riggs, the scientist who discovered the mistake, certainly didn't think so:
"As the term 'Apatosaurus' has priority, 'Brontosaurus' will be regarded as a synonym."
Stephen Jay Gould, in his essay Bully for Brontosaurus, didn't think it mattered much either:
"If you . . . [claim] that our postal service has mocked the deepest truth of paleontology, I will know that you have only skimmed the surface of my field."
Basically, it's good to know that Brontosaurus is actually Apatosaurus, but it's also good to know that it doesn't actually matter.