I remember some website having them as a gag. It would show you some complex math equation but the answer was always 0. If you failed enough, because you didn't

I remember some website having them as a gag. It would show you some complex math equation but the answer was always 0. If you failed enough, because you didn't get the joke, you'd get a (1 - 1) or something simple that still worked out to 0. Sometimes you'd get a [zero] as a regular captcha. I don't even remember what the site was, or was about, but I still remember that captcha.