We previously wrote in detail about how botnets hammering a web site can cause outages.
Here is another case that emerged in the past month or so.
Again, it is a distributed attempt from many IP addresses all over the world, most probably from PCs infected with malware.
Their main goal seems to be to add content to a Drupal web site, and trying to register a new user when that attempt is denied because of site permissions.
The pattern is like the following excerpt from the web server's access log.