Full Idea
It would be irrational to obey even the most useful rule if in a particular instance we clearly see that such obedience will not have the best results.
Gist of Idea
For consequentialism, it is irrational to follow a rule which in this instance ends badly
Source
Philippa Foot (Utilitarianism and the Virtues [1985], p.62)
Book Reference
Foot,Philippa: 'Moral Dilemmas' [OUP 2002], p.62
A Reaction
This is the simple reason why attempts at rule utilitarianism always lead back to act utilitarianism. Another way of putting it is that a good rule can only be assessed by the outcomes of individual acts that follow it.