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.
The Scratch 404 page did not always have an image on it. In December, 2009, the then-Scratch Team member andresmh asked for an image to adorn the 404 page. The chosen image was made by the Scratcher DavidTy. He was credited with the creation beneath the image on the page.