Google, yahoo and MSN has come to a conclusion on Canonical Tag.Search engines will consider this as a Guidence to them, not as a directive, Unlike “No Follow Tag”

Reason for 404 Error Pages * A mistyped URL, or a copy-and-paste mistake * Broken or truncated links on web pages or in an email message * Moved or deleted content Confronted by a 404 page, users may then attempt to manually correct the URL, click the back button, or even navigate away from your site.