The best example after the Parallel Postulate may be a conjecture by, of all people, Fermat. He thought all numbers of the form 2^(2^n))+1 were prime. http://en

The best example after the Parallel Postulate may be a conjecture by, of all people, Fermat. He thought all numbers of the form 2^(2^n))+1 were prime. http://en.wikipedia.org/wiki/Fermat_number http://en.wikipedia.org/wiki/Fermat_number However, I'm not aware that he ever claimed to have proved the result, "wonderfully" or otherwise. :)