If for every rule there is an exception, then we have established that 
there is an exception to every rule.
If we accept "For every rule there is an exception" as a rule, then we 
must concede that there may not be an exception after all, since the rule 
states that there is always the possibility of exception, and if we follow 
it to its logical end we must agree that there can be an exception to
the rule that for every rule there is an exception.