by Chadwick Wood • November 23rd, 2006
The idea is to record every visit to a missing page as the address of the missing page, but prepended with a folder name. Once you do this, in Google Analytics you can view "Content Drilldown" (Content Optimization -> Content Performance -> Content Drilldown) and you'll see a folder with the name "notfound". Inside that folder will be all of the missing pages that your visitors looked for.
For more information on 404 pages and how to make them very nice, there's a great article at A List Apart.