Cute 404

I'm trying, really TRYING to grok this whole sed thing. Some in place text replacement, a little auto-editing, that's all I'm looking for. On one tutorial, there was a link to some more information, but that page was missing. But that was OKAY because the 404 error message was so cute. Go on, take a look at this:
http://www.softlab.ntua.gr/unix/docs/sed.txt