CAPTCHA
From SDWiki
A CAPTCHA, or Crappy hard-to-read text in an image that you have to type, is an inconvenient and poorly-thought attempt at reducing message board spam. Theoretically, it works because humans can read the image and type the text it shows, but bots can't. Unfortunately this logic doesn't work because a machine had to generate the images to begin with, so it's fairly trivial for someone with a neural net and some experience in AI and image processing to crack most CAPTCHA images with at least enough accuracy to crapflood a board anyway. So why do CAPTCHAs still work? Simple. For the few hours it would take to write a CAPTCHA cracker, a spammer could have spammed fifty forums that don't use one.
Not to mention accessibility issues. They're hard to read by definition, so normal people often have trouble with them. This means people with some difficulty reading otherwise readable text are twice disadvantaged. On top of that, blind people can't use them at all, because screen readers can't find anything to read. You can't just put the text in the alt= attribute, because that would defeat the point. An increasing trend is to have an audio link beside it so blind people can listen to it. This is also stupid. Suppose someone is also hard of hearing -- now you have two problems. Oh, and who knows if the browser is properly set up to begin with; maybe it won't know what to do with the audio file and you'll just end up downloading it to some temporary directory and not hearing anything... or crashing the browser.
On top of all that, CAPTCHAs don't prevent humans from posting spam or unwanted messages, and otherwise generally crapping up the board. No matter what you use, if your website is in any way dynamic, you need moderators to keep the place clean.
