Expectations Alter Outcomes
A great article by Sharon Begley in the Nov. 7 Wall Street Journal discusses the affect of teachers' expectations upon students learning. In a typical experiment, elementary school teachers are told that a certain did well on a test that predicts intellectual "blooming". After a few months, the test seems proved right: all the labeled bloomers have done much better than the other students.

Except that there is no such test. The labeled students were picked from every ability level (as measured on standard intelligence tests).