I vaguely remember that at least one version of the 6502 had half a mul instruction, that is, an attempt was made to add one, but it was buggy: it only worked for some multiplicands (ones without any neighboring one bits or something like that). That made for interesting programs, where, depending on the arguments one would compute a times b, b times a, a1 times b plus a2 times b for some a1 a2