I had a chemistry teacher who insisted that the fissile isotope of Uranium was U-238 not U-235. I challenged him on this multiple times and he refused to budge on this. I get that it's a simple mistake to make (it seems like U-238 is bigger so intuitively ought to be less stable) but he could have just looked it up and he didn't, I guess he was just so confident about it that he thought there was no way he could have been wrong about it.