I recently been given a site that has over one-hundred thousand 404 error codes listed in Google Webmasters. It is really odd because according to Google Webmasters, the pages that are linking to these 404 pages are also pages that no longer exist (they are 404 pages themselves). These errors were a result of site migration that had occurred. Appreciate any input on how one might go about auditing and repairing large amounts of 404 errors. Thank you.

In what type of a situation is it the best type of practice to use a self referencing rel "canonical" tag? Are there particular practices to be cautious of when using a self referencing  rel "canonical" tag? I see this practice used mainly with larger websites but I can't find any information that really explains when is a good time to make use of this practice for SEO purposes. Appreciate all feedback. Thank you in advance.