From BNET:

Favoritism in the workplace is worse than you think. A new study shows that when it comes to promotions, bosses' gut instincts matter a whole lot more than any performance reviews.

Specifically:

Most bosses already know who they want to promote before any review process takes place. Some 56 percent of bosses already have a 'favorite' in mind for a promotion before the formal review process begins.

The favorites get promoted. Once the review process is over, the predetermined favorite gets the promotion a stunning 96 percent of the time.

Favoritism is widespread. Some 75 percent of the survey respondents say they have witnessed favoritism, and 23 percent admit they practice it. This is especially interesting, since 83 percent say this sort of unfairness leads to worse decisions in promoting people. In other words, some people are practicing favoritism and not even fooling themselves that it's a good idea.

Personal note: in civil service positions, where the favorite has to place in the top three ranks to be promotable (if there is such a word), coaching by the manager or supervisor to do well in the exam is common practice.

I remember once being directed by the manager to "tutor" a candidate she wanted to promote. I had taken the test and aced it. When I refused, she made it clear I myself would never be promoted again. I was not a "team player". Luckily, I never was. I would have spoken out and been fired for my honesty. Or, if not fired, given a ration.