The robots.txt file is small and powerful, and easy to get wrong.
Blocking everything
Disallow: / under User-agent: * asks every crawler to stay away from every page. It is often left over from a staging site.
Blocking CSS and JavaScript
Search engines render pages. If they cannot load your styles and scripts they may misjudge your layout.
Using it for privacy
Robots.txt is public and only a request. Use passwords or noindex for private pages.
Forgetting the sitemap
Add a Sitemap line so crawlers find your pages quickly.