GoKawiilVulcan: the 19th-century 'planet' astronomers wrongly confirmed for decades
In the 1800s, astronomers proposed a planet called Vulcan orbiting the Sun inside Mercury's orbit, partly inspired by the successful prediction of Neptune based on anomalies in Uranus's orbit. Vulcan was reportedly sighted multiple times over roughly forty years and its properties were described in detail, even though it is now understood not to exist. What observers actually saw remains unexplained.