Missing canonical
OpportunityLow priority
Parsed pages that declare no canonical, in the head or in a Link header.
Why it matters
Without one, a search engine picks for itself which of several addresses for the same page to keep, and may not pick the one you would.
How to fix it
Add a canonical in the head naming the address this page should be known by, which is usually itself.
Skitter runs this check on every crawl, and lists each page it finds with the evidence: the header, the tag or the link that gave it away.