Cookies for Comments is a WordPress plugin that can be used alongside Akismet to significantly reduce or even kill completely comment spam.
Version 0.5.4 adds a rejections page in case human visitors trip over the cookie check. You can modify the message shown to visitors and it’s obviously only useful if you don’t use the cookie check in your .htaccess file.
It happens sometimes, usually when the visitor has cookies turned off, but very occasionally for some other reason. (That I don’t know of yet)
This plugin isn’t a replacement for Akismet as it is easily bypassed. Thankfully 99% of spam bots don’t bother checking so this plugin works quite well!