Thousands of billboards at railway stations across the Netherlands went blank by law after a court ruled that the country’s train operator, NS, had broken European competition rules. The NS subcontracted its advertising space to two Dutch firms rather than opening it up to bids from across the EU. But all that empty space gave Publicis an idea. In a guerrilla-style campaign it stuck pictures of Aquafresh whitening toothpaste tubes on the white billboards. Of course NS employees soon peeled them off, but in the meantime the stickers made a lot of customers smile.