Here's an example regarding canonical URLs and duplicate content:
Directory form, www.google.com/product(/)
www.google.com/product (canonical), try version:
with: suboptimal behavior when trailing slash added
* includes product main pages in directory form without a trailing slash
200 status code given when slash added to Google Products' canonical URL, Sept. 2009:
Avoid multiple URLs that serve the same content. From the example above, the good news is that visitors will reach the content no matter which version of the URL they choose. This is because a "200 OK" status code is given for both URLs. The bad news is that each of these URLs will get crawled and indexed by search engines, creating duplicate content. Search engines will have a tougher time deciding which URL is the canonical. Also, each URL will have its own reputation. Using a 301 on www.google.com/products/ will consolidate this valuable reputation so that the canonical can rank to its fullest.
404 status code given when slash added to Google Finance's canonical URL, Sept. 2009:
Prevent 404s. A lot of visitors will try to reach Google Finance with the URL finance.google.com/. Many others will try www.google.com/finance, but a large number will also try www.google.com/ finance/, which leads them to an unhelpful 404 page. Some visitors will assume that the service is down ("Why wouldn't www.google.com/finance/ work?"). Others might try another form of the URL, but say, "I never know which URL to choose for Google's products!" Think of the most common URLs that visitors might try in order to reach your product, then 301 redirect these to the canonical URL. This will prevent a lot of frustration for users who access your product by typing the URL in their browser's address bar.
404 page shown when slash added to Google Finance's canonical URL, Sept. 2009:
To see the whole shebang, download the SEO Report Card