I've heard of it happening...and really when it comes to court it's all about perception. All a lawyer has to do is put doubt in the minds of the jury and anybody who was just defending themselves can be seen as a cold blooded killer.

I tried to find precedence on the interweb and couldn't find anything but I did find some interesting links while looking:

http://teapot.usask.ca/cdn-firearms/Buckner/Chapter%2010.htm
http://www.self-defender.net/law5.htm
http://www.canadagunlaws.com/self-defense-laws-in-canada/