“Be conservative in what you do, be liberal in what you accept from others.”1)
Relations to Other Principles
Fail fast (FF): While FF is (amongst others) about checking for erroneous parameters, Postel's Law is about not being too strict with parameters. It says that the design should allow for uncommon or strangely arranged (yet meaningful) input data. This does not contradict FF as Postel's Law does not demand to process meaningless or erroneous data.