A while into one of my previous positions, I was reviewing some of the code behind our main web site, all hand coded by one of my colleagues.
As I came to the contact us form, I noticed the typical spam blocking technique of asking the user to do a small mathematical equation... you know, the normal "What's 2 + 5?" type question.
Interestingly, the form's submit button was not enabled to begin with. As soon as the equation was answered correctly, however, the button became active and the user could submit the form.
Taking a quick look at the code in the browser, it was obvious that this form was entirely using JS to 'enforce' the spam protection. That is to say, JS was required to enable the submit button, and there was no actual server side validation of the spam protection.
Knowing that this is really not the way to do this at all, I asked the developer about it. His answer was that "of course it works... we haven't been getting spammed, have we?"
The answer reminded me very much of Homer Simpson buying Lisa's tiger repelling rock.
Anyway, I tried to make headway with the developer, trying to discuss bots not usually running JS, and so on, but it made no sense to them... as far as they were concerned, it worked as expected.
I finally realised I was just hitting my head against a brick wall and continued with the rest of my work. It was true, we weren't being spammed, and if worse came to worst I could resolve it quickly myself if ever we began to be spammed.
The developer left us not long after, but it was before another colleague and I discovered Google's Skipfish. Without thinking too critically (though we were sure to make sure we weren't going to try anything destructive!) we set it on the site. Things were going really well, until it found our contact us form... and hit it with 100s of different tests. That was the day that we found out that the contact us form also did something else interesting: it sent the email to about a dozen different staff members. By the time it was stopped, I think they each got around 1000 emails, which was really embarrassing.
But at least, I guess, we weren't attacked by any tigers.