We're not condoning this or admitting to having done it (yet), but a colleague of ours has confessed to hiding a single leaf from one plant in another plant at the nursery, paying for one plant, and rooting the single leaf she nabbed at home. She's a paying customer, in one sense, but she's also secretly smuggling out some goods.
It's just a leaf. So would you do it? Is this a common practice? How about plucking a leaf from a neighbor's plant without their knowledge?