I'm currently reading the book Straight and Crooked Thinking by Robert H. Thouless. Its a slim little volume, but exceptionally weighty in calm and rational wisdom.

"The exception that proves the rule" is a wisdom often used glibly to 'prove' any weak argument.
Its very often used by those who supposedly posesses more worldly knowledge than their opponent. There is the implication that the user has had more real world experience in the matter at hand and therefore has more of a 'feel' for its intangibles.

He is qualified, therefore, to override all the intervening logical progressions and declare immediate victory by means of superior knowledge.

When you think about it for more than a couple of seconds, its pretty obvious that it isn't only just wrong. Its validity completely dissolves the logic upon which a rational argument might be based.