404 error
A 404 code indicates that a page does not exist at the requested address. A handful of 404s are normal in any site’s life; hundreds of internal or external links pointing to 404s waste authority and crawl budget.
In practice
Triage by value: a dead URL that had external links pointing to it merits a 301 redirect to the closest equivalent; one with no history can remain a clean 404. The 404 page itself is worth designing — internal search, links to main sections — to retain the lost visitor rather than sending them back to the search engine.