Earlier this week, Allan Scott of Google posted a blog post on the Google Webmaster Blog named 5 common mistakes with rel=canonical.

This is really a must read for any developer, webmaster or site owner with a larger site.

In short, the top five canonical mistakes are:

(1) rel=canonical to the first page of a paginated series

(2) Absolute URLs mistakenly written as relative URLs

(3) Unintended or multiple declarations of rel=canonical

(4) Category or landing page specifies rel=canonical to a featured article

(5) rel=canonical in the body>

I recommend you read the full blog post, which has illustrations to explain all of it.

Forum discussion at Google+ & WebmasterWorld.