A 404 Not Found error is an HTTP status code returned when the server cannot locate the resource at the requested URL. This is one of the most recognizable error codes on the web, encountered by users when they follow a broken link, mistype a URL, or access a page that has been removed without a redirect.
In URL shortening, 404 errors occur when a user clicks a short URL whose code does not exist in the database. This can happen because the code was never created, the link has expired, or the link was deleted by its owner. Well-designed shortening services provide a helpful 404 page that explains the situation and offers alternatives, rather than displaying a generic error. UX design books on Amazon discuss error page best practices.
From an SEO perspective, 404 errors are not inherently harmful, but a large number of 404s can indicate poor site maintenance. Google Search Console reports 404 errors found during crawling, and site owners should regularly review these reports to identify and fix broken links or set up appropriate redirects.
The distinction between a 404 (Not Found) and a 410 (Gone) is important. A 404 means the server does not know whether the absence is temporary or permanent, while a 410 explicitly states that the resource has been intentionally removed. For expired short URLs, a 410 response is more semantically correct. Web standards books on Amazon explain these nuances.