How do you log URL's of 404 page not found?
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I've search but haven't found....

I recently created a new layout for a website. It is live now. Traffic seems to be going down. I am afraid that I forgot to move some of the pages over. Is there any way to log url's that people go to but doesn't exist?

For example:
If Joe goes to www.sample.com/page3
and if that page doesn't exist, I want it to log the url in say error.log on the server

Please advise and thanks in advance.