I recently uploaded a customized not found page to my site and I thought it was working correctly in that it contained a link to my home page which seemed to work. I don't recall if I actually used the link or not however last night I did by requesting a non-existent page which produced the not found page however I discovered that the link was attempting to take me to the requested page which didn't exist as I had just made up the name.
I checked the link on the page on my computer and uploaded it again to my root directory.
I then entered a request for a non-existent page in Firefox and the new not found page appeared however the link had changed to that of my requested page.
After submitting the above post I was messing around with the .htaccess and notfound.shtml files when I decided to try adding "http://" in front of the "www.mydomain.com" to see if this would work.
I uploaded the file and am pleased to report that including the full URL appears to be necessary as the page is not loading correctly with the correct link i.e. my home page - not the page that could not be found in the first place.
I think I got the information from the following link which is correct but does not mention that the full URL is required: