series and the exception rule

There is a rule about exceptions in a series.

Which is, as soon as you allow one, you will shortly riddle your rule with them.  It's just too easy to allow yourself to do it again, and again, and again. . . .

Which is a real pity because the drama of this it doesn't work that way this time makes a great climax.  Until it gets undermined because its value was its uniqueness.

Doesn't matter what the rule is.  Nobody comes back from the dead; everyone must sacrifice blood to work spells; this stoic character never shows emotion.

The real irony is when it is done (almost) right as a new discovery or development.  'cause it often seems that as soon as the new rule is propounded, everyone knew it, and always knew it.  And it never spreads or even has serious arguments against it.  (Raising the question of why the old rule lasted so long, if there were no arguments for it.)

Suppose the only way around it is not to allow the exceptions, unless you've got the time to make them central, and show the development.  sigh