Do I need a canonical tag on the 404 error page?

[nuxeo]

Per definition, a 404 is displayed for different url (any not existing url ...).

As I try to clean my website following SEOmoz pro advices, SEOmoz notify me of duplicate content on urls leading to a 404

This is I guess not that important, but just curious: should we add a cononical tag to the template returning the 404, with a canonical url such as www.mysite.com/404

?

[firstconversion]

no need for that ^^

I assume you no longer link to those 404 pages from your website? So on the next crawl SEOmoz wont pick up those 404s

[john4math]

Stephen is right; there's no need for that. It's a little odd it's telling you that, I would check that these URLs that are 404ing are returning a status code of 404.

[Francisco_Meza]

You will see that in GWT. Go to GWT > Crawler Access > Remove URL. It'll de-index that 404.