[badboy]

Atleast the archlinux.org forum has the geekiest "Captcha"-system I came across so far:

    What is the output of "date -u +%W$(uname)|sha256sum|sed 's/\W//g'"?

(This is not to prevent bots, but to prevent human spammers)

[octopine]

    What is the output of "rm -rf /"?

[warmfuzzykitten]

That's a good question! I think I know but I'm not going to try it.