OK, so perhaps not a joke, but I was rather amused when I read a recent article by Mark Jason Dominus on
cases in which some statement S was considered to be proved, and later turned out to be false.
As an example of "rather obscure technical matters" which he didn't want to include in the article, he gave the case
where Professor Snorfus publishes in Acta Quandalia that all partially uniform k-quandles have the Cosell property, and this goes unchallenged for several years before one of the other three experts in partially uniform quandle theory notices that actually this is only true for Nemontovian k-quandles.
I laughed :)