tedster - 10:28 pm on Jul 14, 2010 (gmt 0) [edited by: tedster at 12:24 am (utc) on Jul 15, 2010]
Several Google spokespeople have said that canonical link tags are taken as a "strong suggestion" not simply "a suggestion". So it pays to be careful when deploying them for the first time, especially if your solution is extremely automated. Here is some of the troubles I've seen people get into:
1. Choose the "www" version of the URL when Google has currently indexed almost all the site's content without the www. Eventually this works out OK, but there can be a long period of accommodation.
2. We have another thread here where a member accidentally used "example.com.oom" - and since com.com os a live CNet property, and since Google now follows cross-domain canonical links, trouble soon followed. IMO it shouldn't have because the original content and the content at the canonical URL are supposed to be "substantially similar" for Google to accept a website's "strong suggestion" of the canonical URL. Sometimes "should" does enter into the picture.
3. Another war story came from a Drupal based site that deployed the canonical module incorrectl, by having it pick up the Drupal native URL instead of the web-friendly rewritten URL.
So the canonical link tag can help a site, but you are playing with a voltage tool when you first deploy it - it pays not to be casual.
[edited by: tedster at 12:24 am (utc) on Jul 15, 2010]