I have just upgraded to 2.8.4 using the standard approach and while checking the site, I found the 404 page behaves peculiarly:
http://www.oldracingcars.info/dufflink
That looks like the bottom of the sidebar followed by the footer.
I'm using the standard 404 page and the standard .htaccess but my own theme. All the other pages work fine.
If I use the default theme I get a similar problem but instead of that piece of HTML I get (in Chrome):
This web page is not available.
The web page at null might be temporarily down or it may have moved permanently to a new web address.
More information on this error
Firefox showed me a blank page and Safari told me the server was busy.
What have I done wrong?