A 404 page should tell visitors the page was not found and offer a useful route back, such as site search, main categories, or homepage. A form error should identify the field that needs correction and preserve already completed information when possible. Avoid blaming the user or showing only an unexplained error code.
Google’s AdSense approval guidance includes broken links and difficult navigation among potential issues. An effective error state reduces dead ends and gives the visitor a next action. It does not mean every removed URL needs to be redirected to the homepage; use a relevant destination where one exists.
Example: A contact form fails because an email field is invalid. Show the field and explain the expected format, then allow the user to resubmit. Test with valid and invalid input.
Use SiteReadyScore to find public errors and URLs that may need checking. Then trigger the actual error state yourself and verify the recovery path.