[math-fun] |A|+|B|=|C|

Does anyone know of any widely-promulgated bogus assertions of the form “The cardinality of set A plus the cardinality of set B equals the cardinality of set C” (where the C is something like the union of A and B) that sounds convincing until you step back and realize either (a) there are elements of C that are elements of neither A nor B, or (b) there are elements of C that are elements of both A and B? Here I’m talking about flimflam at the interface between math and culture (not mistakes made by mathematicians in a mathematical context); e.g., fallacious stuff said by public figures. Jim Propp