I have asked this question of seven mathematicians. From six I have received the same “answer”: “There must be one, but offhand I can’t specify it”. The seventh said “maybe it’s never happened”.
I know of a number of mathematical “proofs” that have been accepted for a good while only to be later shown to be false. However, in every case I know of, it was the reasoning, not the conclusion, that was shown to be incorrect. I do not know of even one case in which the conclusion was not later proved to be correct. For example, "Iron-clad proofs" of Euclid contained unrecognized assumptions that made the “proofs” to be not proofs. However, Hilbert later proved Euclid’s conclusions to be correct.
Do you know of a “proof” that was long accepted and then demonstrated to have an INCORRECT conclusion?
NOTE: I know of a number of near misses: A book by Girolamo Saccheri published in 1773, was not widely accepted. Mersenne gave a list of exponents he believed generated Mersenne primes, but it was an incorrect list,not a proof. Riemann believed that his analysis of the frequency of prime numbers showed
that the integral of the logarithmic function remains larger than the prime number counting function for large n. This turns out not to be the case. But it was not a proof, but a speculation