I used to have fun filling in those reCAPTCHAs with incorrect answers. They would show you two words, and it was always obvious which word was the actual CAPTCHA test and which was the OCR input, because the former would be warped into a funny shape while the latter was always a rectangular block of normal text. So I'd type in the correct answer for the warped word, and something like "fuckface" or "cocksucker" for the regular text, and it would be accepted.

I like to think that somewhere out there on Google Books, my efforts have resulted in an innocuous word being replaced with something offensive.