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.
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.