It can be said that the history of humanity is an ever-increasing knowledge. Overall, we now know more about many things than people in the past centuries. This does not mean, however, that there were no missteps along the way.
One could not believe that it would be so easy to "discover" something that does not actually exist – because it is not the case. Many people throughout history have claimed to have discovered things from which were later shown to be unreal, be it mythological creatures, dinosaurs, moons, islands or even elements.
January 8, 1493 In the year 1493, Christopher Columbus discovered three mermaids swimming in the sea when he made his first trip to America. He wrote in his diary that he watched the three mermaids rise from the sea. He added that they were not as handsome as people claimed and had some male features.
Columbus was not the first sailor to claim the discovery of mermaids. Sailors of the day often stated they had seen real mermaids sailing across the ocean. Today we know that these so-called mermaids were actually manatees, a large marine mammal that can be considered remotely to be human or humanoid.
It is known that the manatee and his closely related cousin, the dugong, emerge from the sea water on their tails as if they were mermaids on the surface. They also have five sets of finger-like bones on their forelimbs and can turn their heads like humans. It's no wonder sailors believed they saw mermaids when they saw manatees. 
The periodic table is one of the most important parts of chemistry. The table is structured so that chemists already know some facts about each element that might be present in the periodic table, even if it was not discovered. In fact, they even reserve spaces for these undiscovered elements.
The Periodic Table had many unfilled rooms decades ago, and chemists often searched for missing elements. Their reward was the exclusive naming rights for every item found. Some named the elements by themselves or by the city in which they lived and worked.
Many scientists, however, often reported discovering elements that did not turn out to be real. Some did this by mistake after confusing some of the bad elements with new ones. Others were after the naming rights of the elements. Later it became a kind of fad. Chemists claimed to have discovered a nonexistent element, and competing chemists to refute the existence of that element.
In 1926, Luigi Rolla of the University of Florence in Italy announced the discovery of element 61 of the Periodic Table. He called it Florentium to Florence in Italy. At that time, several chemists were working to find the item, and Rolla quickly announced his "find" because he wanted to be the first.
Later, Rolla's colleague Lorenzo Fernandes called for the withdrawal of the "Discovery." Rollo, however, insisted that Florentium existed and destroyed Fernandes' reputation. Another group of chemists at the University of Illinois claimed that Element 61 was discovered several months after the discovery of Rollo. They called it Illinium to Illinois. Chemist Charles James also said that he found the element.
The existence of Florentium, Illinium or whatever an imaginative name was called, was later refuted. The actual element 61 is promethium, a radioactive element that is rare on Earth. An embarrassed Rollo posted a revocation of the Florentium element in an obscure Vatican journal. He also wrote most of the retreat in Latin so that people would not read it. 
In the late 1800s, the American paleontologists Othniel Charles Marsh and Edward Drinker Cope participated in a contest to discover and join the latest dinosaur fossil to name. This period was called Bone Wars.
Both paleontologists often attempted to announce the discovery of the newest dinosaur in front of the other. At the same time, both men tried to prevent the other from discovering and naming new dinosaurs. There are claims that they have often ordered fossil collectors to destroy remaining fossils in the ground so that others would not find them.
In 1877 Marsh discovered the incomplete fossil of a long-necked dinosaur that lacked a head. Marsh announced the discovery of the new dinosaur in 1883. He added, however, the head of another dinosaur. He named his new discovery the Apatosaurus (deceptive lizard).
Marsh discovered the fossils of a similar-looking dinosaur in 1885. He named him Brontosaurus (Thunder Lizard). In fact, the so-called Brontosaurus was actually a more complete Apatosaurus . Some scientists discovered Marsh's fraud in 1903 and declared the Brontosaurus Apatosaurus .  ( Apatosaurus is shown above.)
was revived as its own genus in 2015, but Marsh's 1885 Brontosaurus was still an example of Apatosaurus .
On August 7, 1869, two scientists, Charles Augustus Young and William Harkness, observed a green line around the sun during a solar eclipse. They made the discoveries independently of each other, which makes it interesting. They regarded the line as an undiscovered element and called it Coronium.
Dmitri Mendeleev, who would later create the periodic table, believed the element was lighter than hydrogen, and suggested naming Newtonium. Today we know that hydrogen is the simplest and easiest element in the universe. We also know that Coronium does not exist.
The existence of coronium was disproved in the 1930s when Walter Grotrian and Bengt Edlen reported that the green line was actually caused by heavily charged iron. Young and Harkness had confused iron with a new element. 
6 Triceratops Or Torosaurus  Triceratops and ] Torosaurus are two other genera of dinosaurs that are controversial are. Both dinosaurs were generally considered to be different but closely related creatures until two researchers suspected that they were indeed the same.
Triceratops (top left) and Torosaurus (top right) have three horns on their heads. However, the horns and some facial features look a bit different, suggesting that they may be different creatures. But then paleontologists John Scanella and Jack Horner claimed that Torosaurus actually grew up Triceratops . Their horns and faces look different because they have changed slightly with the growth of the dinosaur.
The announcement caused great consternation among fans of the more popular Triceratops . They feared their beloved Triceratops would be renamed into less cool-sounding Torosaurus . The dinosaur, however, would most likely still be called Triceratops if the two genera were the same in the end since Triceratops was first named. 
Even the claim that Triceratops and Torosaurus are one and the same remains unproven and has been highly contested in paleontological circles.
Neith used to be known as the Moon of Venus. The discovery was reported on January 25, 1672 by the famous astronomer Giovanni Domenico Cassini. Cassini said the moon was a quarter of the size of Venus. He also reported having rediscovered it on August 28, 1686.
Cassini's discovery was supported by James Short, who discovered the moon on October 23, 1740. He said that he had one-third the size of Venus or less. The existence of the supposed moon quickly became shaky when several prominent astronomers claimed they could not find it, even after other equally prominent astronomers said they had seen it.
The existence of Neith was partially refuted by the head monk of the Vienna Observatory in December 1757. The abbot reported that he had seen the moon only with one of the several telescopes in the observatory. The other telescopes had nothing planned. He added that he felt the supposed moon as a comet.
The monk discovered in March 1758 again the elusive moon. He said the moon was a moon, it was nothing. He concluded that Neith was indeed an optical illusion. In 1887, Mr. Paul Stroobant claimed that the observations of the "moon" were either stars or caused by defective telescopes. 
And now back to nonexistent elements that humans "discovered". In 1864 Sir William Huggins announced that he had observed an element he called Nebulium. Huggins made his discovery with a spectroscope.
The spectroscope worked with a film or prism that could split light into different wavelengths. It revealed the existence of new elements, as each element emits and absorbs light at different wavelengths. For example, when heated, sodium emits wavelengths ranging from orange to yellow.
Huggins used the spectroscope to separate light from a fog. He discovered that it radiated light in two shades of green. Huggins mistakenly believed that he had discovered a new element because no known element emitted light at these wavelengths. He called it Nebulium after a fog.
The existence of nebulium was refuted in 1927 when Ira Bowen discovered that "nebulium" was actually oxygen, only in a state in which it could never exist on Earth. 
3 Buss Island
The discovery of Buss Island is often attributed to explorer Martin Frobisher, although Captain James Newton discovered it during an expedition in 1578 into the Arctic. This was probably because the expedition was led by 15 ships by Frobisher.
Newton, the captain of Emmanuel one of the ships of the expedition, announced the discovery of the new island as they sailed past Southern Greenland. The island was named after the Emmanuel which was a bus ship: a kind of small fishing boat with two or three masts.
Strangely enough, other ships regularly passed the area, and no one had discovered the area of the island. However, no one asked Captain Newton, and the cartographers added Buss Island to their maps. Other sailors soon reported that they had not seen the island, even after careful attention as they passed their supposed location.
This changed in 1668 when Captain Zachariah Gillam reported seeing Buss. A few years later, Captain Thomas Shepherd of the Hudson Bay Company claimed to have discovered and even landed on the island. Captain Shepherd made a map of Buss Island and named parts of it after the 12 executives of the Hudson Bay Company. The Hudson Bay Company later received a royal charter granting it the exclusive rights to everything found on the island.
The Hudson Bay Company sent a Captain Shepherd-led two-ship expedition to explore the island. Captain Shepherd could not find it. Company executives were so devastated that they did not want to find the island again. Nevertheless, other ships unsuccessfully searched for Buss Island. Buss Island was finally found to be non-existent in the 18th century. There have been suggestions that it may have fallen due to volcanic activity. However, a search of the seabed revealed that no island had sunk there. Other suggestions suggest that it was a mirage or that a known island was mistaken for a new island. Some even think that the captains considered part of Greenland to be an island. 
In 1861 the German astronomer Hermann Goldschmidt announced the discovery of Chiron at the ninth Saturnmoon. Other astronomers, however, doubted the existence of the moon because they could not find it. Only in 1977 Charles Kowal discovered the elusive moon. Kowal observed that Chiron was indeed a small body circling between Saturn and Uranus.
While Chiron 2060 as a centaur (a body orbiting between asteroids and Kuiper belts) is now considered a "moon," his earlier "discovery" as a moon led to the observation of the true ninth Saturn moon. The moon Phoebe was discovered in 1898 by William Henry Pickering. Pickering discovered Phoebe after an analysis of the observations Goldschmidt made during his "discovery" of Chiron
In 1905, William Henry Pickering announced the discovery of Themis, the he thought he was the tenth Saturnmoon. The problem was that Themis did not exist like Chiron. In fact, no other astronomer was able to find it. However, this did not prevent the French Academy of Sciences from awarding Pickering the Lalande Prize for the discovery of Themis.
Themis was later uncovered as nonexistent. Janus, the true tenth moon of Saturn, was discovered in 1966. It is very unlikely that he is Themis because they have very different orbits. Pickering spotted his so-called moon circling between Titan and Hyperion, two other Saturn moons. Janus circles very close to Saturn.
Pickering made strange "discoveries" after Themis. In 1921, he claimed to have observed growing plants on the moon. He added that the shape of the Eratosthenes crater of the moon was caused by some insects living on the moon.
Nevertheless, Pickering made several important contributions to astronomy. In 1919 he predicted after Neptune the existence and location of an undiscovered planet. His prediction was crucial to the discovery of Pluto.  In 1908 he also predicted that one day the aircraft would be used to drop bombs.