There are a few common misconceptions related to 404 pages and SEO that I’ll attempt to clarify.
While it’s common (and technically correct) to call 404 pages “errors,” I am not a fan of that terminology. 404s are the expected result when a website is unable to return a given request, if there is no URL to deliver. If we wanted to be picky about it, we’d say 404 pages are not errors and can even be (blasphemy!) an acceptable user experience.