404 Not Found

404 Not Found means the origin server did not find a current representation for the target resource, and it is not disclosing whether the absence is temporary or permanent. It is the internet's most recognized status code, used both for resources that genuinely never existed and, deliberately, as a way to hide the existence of resources a server would rather not confirm, since a server may return 404 instead of 403 for that reason. Because it carries no information about why the resource is missing, well-built error pages and APIs supplement it with a body explaining next steps.

A soft 404 occurs when a page displays "not found" content to a human but the server actually returns HTTP 200, which is invisible to monitoring tools, uptime checks, and search engine crawlers that key off the status line rather than the rendered content. Google Search Console flags soft 404s explicitly because they waste crawl budget and can leave dead pages indexed. For SEO generally, a deliberately removed page should use 410 Gone rather than 404 when the removal is permanent and intentional, and a moved page should use a 301 redirect rather than a 404 followed by a manual link update.

URL normalization is a frequent, avoidable source of 404s: Linux-based servers are case-sensitive by default, so /About and /about can resolve differently even though users treat them as identical, and a missing or extra trailing slash can likewise route to a different or nonexistent handler depending on the framework's routing rules. The fix is consistent server-side normalization, redirecting variants to one canonical form, rather than expecting every internal link and external backlink to match exactly.

Single-page applications routed entirely on the client side are a common self-inflicted 404: the router handles /dashboard/settings fine when navigated to from within the app, but a hard refresh or a direct link sends that URL straight to the server, which has no matching route and returns a real 404. The standard fix is a server or CDN fallback, such as nginx's try_files or a platform-level SPA rewrite rule, that serves index.html for unmatched paths and lets client-side routing take over from there.