One guy does catch on when he learns one more clown (the 1 he was purchased to kill) was explained to to do this to the alarm dude (ideal in advance of capturing him himself for ending his endeavor of opening the vault), but is wrong in who kills him. You could say that he's the perfect henchman, which tends to make the Joker's eventual taking pictures of him just basic erroneous, even for supervillainy.