Problem back. Message below quoted exactly.
Message reads:
404 not found.
/null
(c:\webserver\mudcat\htdocs\null)
Please return to the referring document and note the hyperlink text that led you here. ['referring document' is linked]

At the same time, http://www.mudcat.org/null appeared in the location box.

By hitting the back button three times, I got back in. Yesterday, the /null remained in the location box. Today it did not.

Is this something at Mudcat? No problem with other websites.