tedster - 3:11 pm on Sep 27, 2011 (gmt 0)
Thanks - that's an important caution. See Why Are There "Canonical Disasters" [webmasterworld.com] for other accounts of extended ranking problems after messing up with canonical link tags.
I haven't heard of extended problems recently, so it's possible Google is now able to fix things quicker if you fix things quickly. But it's not an area to mess around with.