Maybe this has been mentioned already, but a quick search didn't turn it up.

I ran into some problems dealing with a Cdn government agency over the phone. Seems the SIN number I gave them "is not an acceptable SIN number"
Took a while before I realized what had happened - for at least the last few years, I have been giving out my AP number when asked for my SIN! (not on my taxes luckily, because my accountant handles those). Both are nine digits, usually spoken in 3 sets of 3.