recaptcha still submits form when one word invalid

Question

Um so I was in for a little bit of a surprise tonight. I spent a good 20 mins trying to figure out why I was able to submit a form knowing that what I entered into the recap

Answer 1

the recaptcha site describes why this is. You need to get one of the two words correct; only recaptcha knows which one.

But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.