g1smd - 10:02 pm on Aug 1, 2013 (gmt 0)
I usually add a noindex meta tag to the 404 HTML file. This is an HTML file (perhaps with some PHP functionality within it) that is "included" after the main PHP script has sent the HTTP 404 HEADER and other things.
I have on occasions had Google directly access the 404 file in the location where it really resides on the server. When accessed this way, the file is natively returned with HTTP "200 OK" status.
In more recent times, I hide these files away somewhere where they can't be directly accessed.