The saga of Carcharhinus leiodon, the smoothtooth blacktip shark, is a very interesting one. The lone specimen was collected in 1902 from the coast of Yemen and not named until 1985. Since there were no newer specimens, some experts doubted the species' validity. Others assumed it was extinct.
Both schools of thought were wrong.
Shark specialist Alec Moore spotted something odd in a fish market in Kuwait in 2008: “amongst the many species of whaler shark was one which looked very similar, but different, to a couple of other species.” The smoothtooth blacktip had been rediscovered and validated.
Fish markets have always been good hunting grounds for ichthyologists, teuthologists, and the like in their search for new species. As the linked article points out, fishermen around the world deploy far grater resources than all scientific vessels put together. They sometime find species that were unknown in a particular location - or unknown altogether. Ask graduate student Paul Clerkin, who found eight potentially new shark species by tagging along with a fishing fleet in the Indian Ocean.
THANKS TO Kris Winkler for pointing me to this item.