The most expensive mistake in SEO fits in one line: a `Disallow: /` shipped from staging to production. It is also the most common, and it is invisible in the interface — nothing on the site looks broken while traffic decays over the following weeks. Check the live file after every deployment that touches infrastructure.
Disallow does not remove a URL from the index; it stops the crawler fetching it, which can leave a previously indexed URL listed with no description. To keep a page out of results, allow the crawl and serve a `noindex` directive instead. Blocking a URL that carries noindex means the engine never sees the instruction.