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Aaseesh from Google responded to a complaint about Google not picking up on a web site's canonical tags. He said that "even if you setup the canonical tags properly, other signals like inconsistent redirects can make Google not select your preferred URL."
Aaseesh said this in a Google Webmaster Help thread and added here's a list of steps you can take to ensure your preferred URLs are getting indexed.
Google's John Mueller said being consistent is the number one rule in SEO. Being consistent with rewrites are also very important, as you would imagine. John Mueller has said this numerous times.
And the above thread is one example of when being inconsistent can have a negative consequence in Google search.
Forum discussion at Google Webmaster Help.
