A 404 error is an error page that is shown when a page requested does not exist or could not be found. An example of a 404 error on Scratch can be viewed here.
404 errors are displayed when one searches for a nonexistent user, or when one searches for a nonexistent or removed project. If one searches for a user that once existed but their account has been deleted, it will return a 404 error to everybody except for the Scratch Team. If a project has been flagged by the logged in user, visiting that project will return a 404 error on the account they reported it with as well. Attempting to visit unshared projects now also returns a 404 error, although this previously had a dedicated page.
The text, "Whoops! Our server is Scratch'ing its head" is displayed, along with a picture of a puzzled Giga with a magnifying glass.