Security Error?

For about the past month, I’ve received the following message whenever I visited the ACLU forums site:

Security Error: Domain Name Mismatch You have attempted to establish a connection with "forums5.aclu.org". However, the security certificate presented belongs to "forums.aclu.org". It is possible, though unlikely, that someone may be trying to intercept you communications with this website."

Recently, I’ve received a similar Security Error message when I visited another web site. Why would anyone try to intercept my communications? It’s almost as silly as saying that the authorities tamper with the mail. Or that they would even arrange for a blog to be redirected so that no one can view it. They wouldn’t do that, would they?