There's a classic case (sorry, cite not handy) involving a day care center that started fining parents for picking up their children late, and discovered to their chagrin that parents became more likely to show up late. The usual explanation for this is that once they were paying for the late-pickup service, they felt entitled to it, and the amount of the fine wasn't nearly as much of a deterrent as the earlier social sanction had been.

The principle seems related: we do different accounting for social and financial capital, and switching a particular task from being measured in one unit to another can totally change the motivation structure for that task.