Back when captchas were about optical character recognition, it was obvious which words came from actual books. Plenty of people would input offensive words instead of the correct answer. I hope it had an effect but it probably didn't.
Someone who isn't me hung around on 4chan a lot back in the day. Indeed, there was an effort across the boards to disseminate heuristics to determine which one was the already-known word and which one was to be parsed by humans. The agreed-upon procedure was identify the latter as "penis".
