I receive this error on my site also. I’ve determined the agent that generally causes this problem and it is a webcrawler like the one used by Google to index pages.
Anyway, I believe the answer is either Cause 1 or Cause 2:
Cause 1
This problem occurs because the client makes an HTTP request by using an HTTP method that does not comply with the HTTP specifications.
Cause 2
This problem occurs because a client makes an HTTP request by sending the POST method to a page that is configured to be handled by the StaticFile handler. For example, a client sends the POST method to a static HTML page. However, pages that are configured for the StaticFile handler do not support the POST method.
Check out the microsoft link for more explanation and possible resolution: http://support.microsoft.com/kb/942051