Canonicals

The canonical tag names the main version of a page. These issues cover missing, conflicting or misplaced canonicals.

Canonical points to a redirecting URL: The page’s canonical tag names a URL that redirects back to this page, so search engines get opposite answers about which URL to index.
Canonical tag missing: The page has no canonical tag, so search engines pick the main URL on their own when the same content is reachable at several addresses.
Canonical tag outside the head: The page’s canonical tag ends up in the body of the HTML, where Google does not accept it, so the preferred URL it names is not used.
Canonical tag uses a relative URL: The canonical tag gives a path instead of a full URL, which is easy to get wrong and can point search engines at an address you did not intend.
Canonical URL contains a fragment: The canonical tag’s URL includes a # fragment, which Google does not support in canonicals, so the preferred URL may not be used.
More than one canonical tag: The page declares more than one canonical tag. When they name different pages, Google is likely to ignore all of them.
Page canonicalised to another URL: The page’s canonical tag names a different URL, telling search engines to index that URL instead of this page.