I've never been happy with this idiom, it doesn't make sense to me at face value. I've always just shrugged my shoulders when I've heard it. 
 
But I was bothered by someone saying it today and decided to look it up. It turns out that it originates from the old meaning of prove - "to test," so that the exception in question tests the rule. 
 
It can also mean an exception the existence of which proves that a general rule exists. Wikipedia gives by way of an example "a sign that says “parking prohibited on Sundays” (the exception) “proves” that parking is allowed on the other six days of the week (the rule)"
 
So glad I've gotten that cleared up! 

Also, the title of this thread is going to be parodied by Bapu. 

But Bapu's parody won't be an exception..

sharke
I've never been happy with this idiom, it doesn't make sense to me at face value. I've always just shrugged my shoulders when I've heard it. 
 
But I was bothered by someone saying it today and decided to look it up. It turns out that it originates from the old meaning of prove - "to test," so that the exception in question tests the rule. 
 
It can also mean an exception the existence of which proves that a general rule exists. Wikipedia gives by way of an example "a sign that says “parking prohibited on Sundays” (the exception) “proves” that parking is allowed on the other six days of the week (the rule)"
 
So glad I've gotten that cleared up! 

But as a rule, I don't think the people who use the idiom in question know any of this, so... 

sharke
Also, the title of this thread is going to be parodied by Bapu. 

And CraigB will say something like "I take exception when you try to rule" .....
 
And George aka "Ham100 and Music-eggs" whyll spall orl da wurdz foe netikly two may kuz larf cuzzitz whell phun knee.
 
Then that old twat Straummy will moan about forum hosts and Doktor Alexlanche.
 
 
etc etc etc
 
 
 
 

 
 
Becan kittens
 
 
Kittens monkeys becan steeeeeenkin badges
 
 
 
 
 
 
 
 
 
 
 
 
 
It
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
There....  I said it
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
becan
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
arse
 
 
 
 
 
 
 
 
 
 
 
 
 
 
I like monkittens, ya 

 
Oscar Pistorius has been granted bail on condition that he wears an electronic tag...
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Not the sharpest tools in the box are the Seth Effrican police... I'd suggest.

 
Becan

I used to get confused by the saying "a moot point" which means it's trivial, taken for granted, not worth debating. And then there are moot contests at law schools- debating teams :) 

sharke
Also, the title of this thread is going to be parodied by Bapu. 

I got nothin'
 
 
 
 
 
 
 
tonight.
 
 
 
Twomarry is an udder day.