This is an ancient topic, but I was just thinking about it the other day.
Do you remember that whole thing about Ultrasaurus?
In 1979, James Jensen of Brigham Young University found what he believed to be the largest sauropod ever. The press went mad and widely publicised the dinosaur under the name Ultrasaurus. I remember as a kid that Ultrasaurus (along with Supersaurus) was always depicted as a huge brachiosaur dwarfing even Brachiosaurus.
However, it took another 6 years before Jensen finally published his findings. By then, a Korean palaeontologist, Haang Mook Kim, had already named a sauropod with the name Ultrasaurus because he thought it belonged to the same genus that Jensen had found. But it turned out that Kim's Ultrasaurus was something different, and when Jensen wanted to use Ultrasaurus, his first preference, he couldn't because it was 'preoccupied'. So he instead named his dinosaur Ultrasauros with an 'o'.
Funnily enough, Kim's Ultrasaurus later became a nomun dubium or a dubious name because there wasn't enough information to assign it to any dinosaur family. Jensen's Ultrasauros had also become a junior synonym of Supersaurus because it turned out that the type of Ultrasauros was in fact a chimera comprised of bones from Supersaurus and a very large brachiosaur. Since Supersaurus was named earlier, Ultrasauros unfortunately had no right to be a recognised genus name.
So despite the huge confusion they caused, both Ultrasaurus and Ultrasauros are now invalid taxa.
Originally posted on DinoBase