If a website does a 301 URL redirect to another URL, that returns a 301 code to the requester, along with the new URL that should be used. That is not an "error". It means that the resource has permanently moved (c.f. 302, which is "temporarily moved"). Search engines will use this to update their index for your site with the new URL. People using a browser will simply see the new URL in the address bar, and can bookmark the new URL in place of the old. Common uses of 301 are to inform others that a site has moved to a new domain, that the directory path or file name has changed, or that www. domain has dropped the www. (or vice versa).