Being wrong is not rare, even for experts. What matters is how a mistake gets found. Science can change an answer when there is new evidence. Pluto was moved out of the planet group in 2006, when the IAU voted for a new definition of dwarf planet. Dwarf planets were defined as objects that orbit the Sun, are nearly round, but have not cleared their orbit of debris. As of November 2026 NASA lists five dwarf planets, including Pluto.

Some mistakes come from what the eye wants to see. Percival Lowell drew maps of canals on Mars from what he saw at his observatory. Later evidence showed they were optical illusions.

Piltdown was a mistake and a fraud together. The fossils were found in 1911 and 1912 and included part of a skull, a jawbone and a few teeth. The jawbone was treated with chemicals to look as old as the skull, and its teeth were ground down. Respected scientists were fooled too. In 1912 there were no good dating methods, so the claim could not be checked in the way we can now.

Why did the mistake last so long? Scientists wanted to find the first human ancestor in England, and that wish may have made them less careful. The Piltdown hoax slowed the search for human ancestors, because other fossils did not fit it. Who made the fake is not known for sure.

In the 1840s a doctor in Vienna, Ignaz Semmelweis, noticed that women whose doctors came from the autopsy room without washing their hands had very high death rates. Before germs were known, high death rates in maternity wards were blamed on forces like polar currents or radiation from the constellations. Semmelweis met strong opposition, and his findings were resisted by hospital and medical authorities.

Columbus is also remembered for a mistake, though not the one in the myth. People knew the Earth was round long before he sailed. Around 200 BC Eratosthenes, head librarian in Alexandria, estimated the Earth's size by comparing the Sun's angle in two places. Columbus sailed using a guess for the size of the Earth that was too small by about a third. A much better estimate already existed.

Some famous stories are mistakes about the mistake. In 1938 Welles's show about Martians was scheduled against a very popular comedy-variety show. Newspapers wrote that many people panicked, but later writers found the panic was much smaller. A survey that called 5,000 homes found that only 2 percent were listening to the Welles play. Radio had taken away advertising money from newspapers during the Depression, so newspapers had a reason to make radio look untrustworthy. Stories about the play grew over time: first newspaper items on October 31, 1938, then retellings.

Sometimes a famous mistake turns out to be a famous test. In 1919 astronomers used a total solar eclipse to test whether the Sun's gravity bends starlight, as Einstein's theory said it would. Einstein's theory predicted starlight near the Sun would shift by 1.75 arcseconds. Eddington's teams measured 1.98 and 1.61. Newton's theory asserted a smaller bend of about 0.87 arcseconds, so the numbers could tell the two ideas apart. The results were published in November 1919.

People look for facts that fit what they already believe. Confirmation bias covers how people search for, interpret and remember information so that it fits what they already believe. It can create self-fulfilling prophecies. Famous mistakes tend to share this. So ask who says it, and how they know, and what would count as a second place that does not already agree.

Words to know

• dwarf: smaller kind
• illusions: false sights
• fraud: planned lie
• survey: set of questions
• bias: leaning