Over at SpamOrHam I use a CAPTCHA from captchas.net to prevent malpeople from using bots to mess up the results of the web site.
There's only one problem with this plan.
People enter the CAPTCHA wrongly about 20% of the time.
Looking at the error logs for SpamOrHam shows that the site has offered 27,468 CAPTCHAs of which 5,326, or 19.39% have been entered incorrectly. I'm not tracking whether incorrectly means that the actual password was wrong, or if the person just didn't even bother to enter anything, but, nevertheless, a 20% error rate is very high.
And for me it manifests itself in complaints and people who give up on the site. That sucks, but it's the current only way to protect against bots attacking the site.
What's needed is a comparative study of the different ways of generating CAPTCHAs to figure out which ones are both effective against bots and effective against humans!