Not an actual 404 error page, but: for a short while, I ran the web site for an Amnesty International writing group in the Netherlands. We were assigned group n

Not an actual 404 error page, but: for a short while, I ran the web site for an Amnesty International writing group in the Netherlands. We were assigned group number 404. The home page had a similar layout to the standard 404 error message but with a text like "Error 404: person not found", and a link to the actual home page. Let's just say people were confused and didn't find us. At least I had fun.