During the course of administering a server full of various sites which have been Farked, Dugg, and StumbledUpon'd, I've learned first hand the value of optimizing a Drupal site/server to handle large amounts of traffic. I've also learned that eventually it's likely that the level of optimization for one's Drupal site/server will be rendered mostly irrelevant by frequent, and (mostly) malicious, circumstances.
Malicious and/or misdirected requests
Compared to more popular subjects such as Drupal optimization, Apache, MySQL, and/or PHP optimization - the subject of malicious requests gets a rare mention. Despite losing the popularity contest to those sexier subjects, rest assured, there is a lot more to running a site than just tuning the former items. All of those things can be working really, really well and your site/server can still be hammered to a state of dysfunction - even with very few users coming through the site.
Welcome to the wonderful world of denial of service attacks and/or server spam. Broadly defined this includes anything that is requesting something from your server that is of a malicious (usually the case) and/or misguided origin. Make no mistake it's a problem which anyone who is running a medium to large size web site will contend with, whether they know it or not.
Server spam/DDS attacks are NOT uncommon problems limited only to large or 'unlucky' websites. If you have a server and/or VPS with a frequently trafficked site, or especially one with several frequently trafficked sites you will be amazed at just how much processor, memory, and bandwidth can be allocated to malicious and/or wrongly directed requests at any given point in time. The exact of amount of resources that these leeches suck up varies greatly depending on many factors, but on the high-end I can share that several times over the last 6 months alone I've personally witnessed, and fixed, crippling issues stemming from server spam for sites that I system admin for. Likewise several months ago I was happy to be able to help resurrect Drupal.org one day when it was suffering from a particularly nasty malbot. (How many people has this happened to that never realize what was going on, and who in turn just blamed their host and/or Drupal??)
So once one realizes that server spam is a real, and not theoretical, problem how does one confirm the issue(s) on their own site/server and what can they do about it?