That's really interesting. I've always wondered why I have passed these CAPTCHAs even when I had to make wild guesses on some of the words because they were so hard to read.
However, how long will it be before a lot of users realize that it is irrelevant what you enter for the unknown word? Even if you don't know for sure which of the word that is the unknown one, knowing the above I think the risk is high that you just type nonsense if you can't read one of the words.
If enough people do this the system will be quite ineffective. reCAPTCHA will probably not accept the wrong solution very often, but it will take a lot of time to get enough users with the same solution to accept it. But with a massive amount of users, even a small amount of the total might be enough to keep it running?