The four points, two distances problem: https://www.theguardian.com/science/2019/oct/21/can-you-solve-it-the-four-points-two-distances-problem

Can you find all of the ways of arranging four distinct points in the plane so that they form only two distances? The link is not a spoiler but it has a separate link to the solution. "Nearly everyone misses at least one" says Peter Winkler; can you guess the one I missed?