5.
Public Site receives a high volume of unwanted traffic
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One source of high traffic can be search robots, which you may or may not want. You may provide a "robots.txt" and use firewall rules to ban requests from IP addresses launching bots not obeying "robots.txt".
Traffic may also be from hackers, attempting to gain access to the site, damage the site, or shut the site down by flooding it with unwanted traffic. Usually the best defense for this is to use a network firewall and/or application firewall. Internet security is a large but well-documented subject and many good resources are available on-line.