Personally, I am not a very big fan of CAPTCHAs, and in some cases, I have given up after a number of tries. While these are in place to ensure that it is a human on the other side and keep spambots at bay, at times they get difficult for even humans to comprehend. But wait, you can never be prepared for what we're going to tell you.
We have CRAPCHA, or Completely Ridiculous And Phony Captcha that Hassles for Amusement, too. So while you will be able to figure out a CAPTCHA (after a couple of tries), you can never comprehend a CRAPCHA; if its makers are to be believed, that is exactly what it is supposed to do.
Its description on Thomas Park, its developer's website, states this clearly. It reads, "CRAPCHA doesn’t serve a dual purpose. It barely serves a single purpose. And it isn’t to keep spammers out."
Meet CRAPCHA. It is CAPTCHA that you can never solve
What CRAPCHA does is annoy the users by displaying a CAPTCHA that they cannot decipher.
You can go ahead and give it a try here. Another fun bit to it is that all CRAPCHA attempts are put up on the CRAPCHA board. Alternately, it allows you to add it to your site by copy-pasting a code. However, as a warning, it says, "CRAPCHA doesn’t actually validate anything. In fact, it’s very likely drive away users, and will cause mixed content warnings on HTTPS. On the other hand, it’s fun."
A little trivia on CRAPCHAs: These are created using a mix of Unicode and Font Awesome characters after they have been put through CSS3.
An acronym based on the word "capture", CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart. The term CAPTCHA made out in 2000 and was coined by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford of the Carnegie Mellon University.
When some CAPTCHAs got ridiculously tough for even humans to comprehend, we got the picture CAPTCHAs. Herein, a user (human) is asked to show a picture of a particular animal from a few animals.
You also have reCAPTCHA, which is doing a noble deed. A free CAPTCHA service, it helps one digitise books, newspapers and old time radio shows.