The MissedPages extension extension helps wikis track the most popular non-existing pages that visitors have attempted to visit. It useful for example when migrating a website to MediaWiki and you want to keep an eye on what URLs have been broken (i.e. are returning a "404 Not Found" message).
The main interface is at Special:MissedPages, where you will see the top 100 missed pages in descending order of how many times they were missed. There is a sparkline chart that gives you an indication of the trend of the traffic to each page title.
For each entry you can take three different actions: redirect, delete, and ignore. These are explained below.
Normal wiki editors can add redirects for missed pages. This involves typing the name of the destination page in the text input; it will autocomplete as you type. When you add the redirect, the MissedPages information for that page will be removed (as it'll no longer be considered 'missed').
This action is exactly the same as adding a redirect to a page manually. If you do add one manually, the page will stay in the MissedPages list until you delete it (although its count will of course no longer increase).
Deleting an entry from Special:MissedPages clears that page title until it is again missed. This can be useful if the page has been created since the log entries were added.
To delete an entry you must have the 'delete' right (i.e. usually that means you need to be a wiki administrator).
If you don't want to redirect a missed page, and you don't want it to ever appear again in the MissedPages log, you can set it to be ignored. This can only be done by users with the 'block' right (again, usually this is wiki administrators).