In addition to my other post about the infamous message from SharePoint: "Updates are currently disallowed on GET requests. To allow updates on a GET, set the 'AllowUnsafeUpdates' property on SPWeb." I came across this error in a rather unusual place and that's the one where you click on a picture in a picture library to show the complete image in the browser.
This occurs if you have the following configuration :
IIS : Virtual server is running in SSL and basic authentication
SharePoint : made a change to the web.config to enable blobcaching (as you can read here to improve the performance of your environment)
When I accessed the environment using Windows Integrated security I did not receive an error. The same applies when I used basic authentication and disabled the blobcache.
Hope this helps someone!