Board Thread:False Info and Speculative Discussion/@comment-24.130.28.230-20150721180826/@comment-95.246.182.32-20150723190852

"exception" means just that : special case of a sure thing. And Banshees are have a lot of exceptionality and only Deaton said it blocks all supernatural creatures, although I do not remember that he has actually said "all of them" but only "blocks the supernatural creatures" without taking into consideration the exceptions. If I'm not mistaken they did not know yet that Lydia was a banshee the first time they used the powder.