Refreshing pages too much
<p>I've noticed in 4chan that if you refresh the page too much (e.g. filling out the form incorrectly too many times .etc) -- they show you the recaptcha page. I'm wondering how this is done, do they use an algorithm or something to decide what is suspicious behaviour? How would you about recording usage history in php (i.e. how would go about doing this in php)?</p>

<p>It would be most likely done using sessions right -- or would they store it in a database using IP address (unlikely due to dynamic IPs).</p>
