I'm not sure if this quite matches your request (I suppose much hinges on the word "important"), but here's a story of a fairly famous and long-standing error:

I'm not sure if this quite matches your request (I suppose much hinges on the word "important"), but here's a story of a fairly famous and long-standing error: https://www.jamesrmeyer.com/ffgit/godel-error-2.html https://www.jamesrmeyer.com/ffgit/godel-error-2.html Personally, I'd say the fact that someone bothered to discover it was false fifty years after it was published shows its importance. The short version is that Gödel claimed in a paper that he had an algorithm to determine satisfiability for a certain logic, but the algorithm actually worked for a slightly different logic instead. People basically took Gödel's word for it for fifty years until a logician called Goldfarb found the mistake. EDIT: As a disclaimer, I don't have any association with or endorsement of the website I've linked to. I found it just now because I needed a source for the story that I've known for many years (I think I first heard it in person, but it's definitely contained in the book `The Classical Decision Problem' by Börger, Grädel and Gurevich). I skimmed the post and it seemed to cover the relevant details, but that's the extent of my knowledge of the site and its contents.