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I just got the "Hello Robot" screen, typed in the two words, noticed I'd mistyped one but my finger hit return too quickly for me to stop, and yet it got accepted! The mistyping was that I missed the last letter of the second word.

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That’s by design – the reCaptcha technique works by displaying two words that were scanned from a book: one of the words had already been recognized, the other not. Thus, reCaptcha checks whether you’re a bot by seeing if you typed the already known word correctly. The other input will be accepted at face value since reCaptcha doesn’t know yet whether it’s correct.

I believe the input is used to train the word recognition software, so with time it gets better at recognizing scanned words.

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    I'd forgotten that one of the words was an unknown; guess I was lucky that it was the second one! Hmm, I hope that I haven't messed up someone else's system then by giving the wrong word. Jul 27, 2010 at 12:02
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    @Andrew: I positively believe that the creators of reCaptcha have already thought of the possibility of someone mistyping the unknown word. Don't worry about it! :) Jul 27, 2010 at 12:53
  • Glad to hear it! I just wish they'd thought of the possibility of an XHTML page wanting to use reCAPTCHA (gripe, gripe). Jul 27, 2010 at 13:02
  • But if they're using reCAPTCHA to make computers better, aren't they slowly making reCAPTCHA less successful at blocking robots?
    – Seamus
    Jun 30, 2011 at 10:25
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    @Seamus Indeed they are. However, I (and obviously also the authors of reCaptcha) feel that the following XKCD comic is highly relevant: xkcd.com/810 – and no, this is not a joke. Jun 30, 2011 at 10:49
  • @Konrad I sometimes think that some of the top rep users here must be actually robots...
    – Seamus
    Jun 30, 2011 at 10:58

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