A page whose content says "not found", but that responds with status code 200. For search engines, that makes it a valid page — and it ends up in the index.

Should deleted pages return 404 or 410?

404 means "not found", 410 means "permanently removed". Both are correct; 410 is the clearer statement when the page is intentionally and permanently gone.

This translation has not been editorially reviewed yet. The German version is authoritative. Deutsche Fassung →

HTTP status codes: does the page even respond with 200?

The check establishes whether the address responds with HTTP 200 at the end of all redirects. Anything else — 404, 410, 500, or a timeout — means there's nothing there for search engines and visitors to get.

The most unpleasant case is the error page with code 200, often called a "soft 404": the visitor sees "page not found", the search engine gets "everything's fine". The result is an index full of empty pages that nobody ever weeds out.
Server errors in the 500 range are especially tricky, because they can occur sporadically. A measurement is a snapshot — the report states when it was measured.
403 also occurs when a protection mechanism mistakes the check for an attacker. In that case, the report states the status code, not a fabricated content finding.

Deutsche Fassung — mit allen Prüfpunkten dieses Themas