I've just discovered something I find quite funny. With a reCAPTCHA, you only need to get 1 of the 2 words right for it to register as correct. Unfortunately, the one you have to get right is usually the most unreadable of the 2.
This is because a reCAPTCHA is made up of one keyword, and one word that has been scanned by an OCR machine (optical character recognition, used to digitise old books) but that couldn't be recognised. So not even the reCAPTCHA itself knows what it says!