How about a captcha where the text image they have to type into the entry field is a screenshot of the How do I post a question effectively? page?

I wouldn't mind seeing the text image be that of: "If you think you're going to use <pre> tags — don't! Use <code> tags instead!" ;-)

However, I largely agree with jvector, who wrote:

I suspect the world is made up of those who read things first and those who don't[...]