Heinz Skunk quoted Witches Abroad by Terry Pratchett
Be careful what you wish for...Once upon a time there was a fairy godmother named …
She heard Nanny say: ‘Beats me why they're always putting invisible runes on their doors. I mean, you pays some wizard to put invisible runes on your door, and how do you know you've got value for your money?’ She heard Granny say: ‘No problem there. If you can't see 'em, you know you've got proper invisible runes.’
— Witches Abroad by Terry Pratchett (Page 57)
Can't argue with that!