A copy of AuthenticatedSystem has been removed from the module tree but is still active!

Hmm, this is a weird one. I just started up script/server today to get
this error when browsing around.

"A copy of AuthenticatedSystem has been removed from the module tree
but is still active!"

Any ideas? I've googled, but the results that show all seem to refer
to Rails 1.*

I don't think I've changed anything that would affect this.

I'm using Rails 2.0.2 and acts_as_authenticated.

Cheers,
Jordan