"Exception that proves the rule"

This is from The Straight Dope, and it's one of my favourite sites.

http://www.straightdope.com/classics/a3_201.html

"I've looked up citations of this saying dating back to 1664, and in every case it was used in the brain-dead manner we're accustomed to today--that is, to suggest that non-conforming cases, by the mere fact of their existence, somehow confirm or support a generalization. Obviously they do nothing of the kind. We like to think proverbs become proverbial because they're true; this one is an exception. It certainly doesn't prove the rule."