I can't remember where I got this link from, but basically: Captchas are solved. In the same way that we have gold-farmers in Romania grinding away at World of Warcraft, there are now multiple data-entry companies in India promising to effectively automate captcha-solving by having masses of low-paid workers manually type them in.
So either your captcha is simple enough that basic OCR software can defeat it, or it's too complex, in which case people genuinely can't get past it, or it's just right at the level where humans can solve it but computers can't - at which point businesses get set up to have thousands of people solve captchas. You can't win.
