There's a great piece on this topic by Joel Sposky in Inc magazine.. Here's a choice snippet:
He's got some great examples illustrating this point. Read the whole thing here.
I'm always on the lookout for these incentive schemes gone wrong. There's a great book on the subject by Harvard Business School professor Robert Austin -- Measuring and Managing Performance in Organizations. The book's central thesis is fairly simple: When you try to measure people's performance, you have to take into account how they are going to react. Inevitably, people will figure out how to get the number you want at the expense of what you are not measuring, including things you can't measure, such as morale and customer goodwill.
...His point is that incentive plans based on measuring performance always backfire. Not sometimes. Always. What you measure is inevitably a proxy for the outcome you want, and even though you may think that all you have to do is tweak the incentives to boost sales, you can't. It's not going to work. Because people have brains and are endlessly creative when it comes to improving their personal well-being at everyone else's expense.
HT: Craig Newmark