How did they do that?

My web-based email at work had this ad (dunno what for, I didn't click through):


How did they get it wrong? "Could you see the two men?" Unless you can see them and say No, or can't and say Yes (and in either case how do they know?) you can't get the question wrong. You can not penetrate the illusion - see only the two old folks instead of the guys sitting on the floor, but how can you get the question wrong?