Is it possible to show which page linked to a page that wasn't found. Some html or php script? Thank you.

Im pretty sure you would have to do this manually, because i think the only way you would be able to do this 100% accurately. Some people customizes error pages so it would be hard to scan for key words and such.

yep with a scripted 404 page

there will be a querystring on the 404 page of 404;url_of_original_request

Okay.