As mentioned in Kevin Casto's comment, this question is almost a duplicate of <a href="https://mathoverflow.net/questions/35468/widely-accepted-mathematical-results-that-were-later-shown-wrong/56964">Widely accepted mathematical results that were later shown wrong</a>. As I mentioned in an answer to that question, R. B. Kershner's paper "On Paving the Plane," <i>Amer. Math. Monthly</i> <b>75</b> (1968), 839–844, announced the classification of all convex pentagons that tile the plane. Kershner said that "The proof…is extremely laborious and will be given elsewhere." The proof was wrong; Richard James III and Marjorie Rice found counterexamples in the 1970s, Stein found another counterexample in 1985, and Mann–McLoud–Von Derau found yet another counterexample in 2015. The classification of all such tilings remains an open problem to this day.