This may be a very naive question, but I can't seem to get the 404 page when I try to access a non-existent page. Instead I get what looks like a fragment of the HTML for the Dashboard!
For example, you can try it at:
(Where 'blah' clearly does not exist)
Just out of interest, I tried 'blah.php', which gives exactly the same result. If you try 'blah.html' the browser tries to render the result, but the result is basically the same.
I'm using a custom child theme based on twentyten, but I've not done anything with the 404 page.
Any help would be greatly appreciated.