Many modern web applications have a lot of spam deterrent such as Captcha, Bayesian filters, URL, ip detections etc. One example is trying to do 2 consecutive search on the CiviCRM.org forum and you will get a an error that look like
"Your last search was less than 5 seconds ago. Please try again later."
The concept behind this is flood control is to prevent a webbot (automated script) that is trying to spam and flood the server.