it's even easier than that. the same captcha gets presented randomly to two people (quasi-)simultaneously. Unless both parties type the same word, neither is verified. This gives an added incentive to people to type what it really says. Of course, it leaves open some room for the sub-class of assholes who will gleefully spend hours typing the wrong answer to captchas, just to fsck with the other guy. That's why you always have to give a failed verification a couple of chances. Hey, sometimes i mistype my password a few times, too. It's no big deal.