Apropos nothing at all…
This phrase is often used to mean the wrong thing. People often use it to mean “the rule must be right, because I’ve found an exception that seems to directly contradict it”. Which is plainly nonsense in many cases.
I had always suspected that the real meaning was “the exception tests the rule”, but it turns out that I’m wrong too.
The real meaning seems to be more like “if you have to make a new rule for an exception, then there’s already an implicit rule for the normal case”.
In the example given, there is a statement: ‘Special leave is given for men to be out of barracks tonight till 11pm’. This statement is talking about an exception (‘special leave’), but it’s also implying that there’s a normal rule that men have to be back before 11pm.
You live and learn…
Update: see also http://en.wikipedia.org/wiki/Exception_that_proves_the_rule