When quality was measured a month later, the team members were given their bonus--a four-week sprint during which they would be their own product owners and could work on whatever they wanted. They took the opportunity to do some refactoring that had been bothering them. One tester took time to explore a new testing tool. Two developers added a scripting interface to a part of the application. This type of bonus was a win all around and avoided the problems that tend to arise with cash or similar bonuses.
Succeeding with Agile: Software Development using Scrum - Mike Cohn, page 29 paragraph 4