In his excellent book Release IT Michael Nygard states that your own marketing department is the most likely source for Denial Of Service (DoS) attacks. In the book he gives a great example of when a big web retailer DoS itself by offering a free XBox 360 for a few selected customers. The customers is given a special code that if entered combined with another purchase will give them the offer. Of course this info is leaked and is published on public forums and web-sites. A few minutes later the load on the web servers sky rockets by requests for a free XBox. The outcome of the marketing effort was a unusable site!
Probably not what marketing had in mind :)
Here is another type of self DoS, this time it's Microsoft who shows us some self denial :)
Thanks to the Hotmail team for inspiring me to this post :)