Jane Elliot conducted one of the most controversial experiments, I would say, in history. Elliot was not a psychologist or scientist; she was an elementary school teacher who wanted to give her students an experiment about discrimination in society.
He divided his students into two groups, one with blue eyes and the other with brown eyes. The teacher then read a series of scientific "proofs" showing that the blue-eyed group was superior to the brown-eyed group. Throughout the day, the blue-eyed group would be treated as superior, while the brown-eyed group would be treated as inferior.
It only took one day for the dynamics of the groups to change drastically; soon the "inferior" group displayed a weak attitude, appearing anxious and nervous, while the other group felt strong and sought to demonstrate this through discrimination.
Elliott's experiment (which he repeated in 1969 and 1970) received much criticism for its negative consequences on students' self-esteem and, therefore, could not be carried out again today. The main ethical concerns would be deception and informed consent, although some of the original participants continue to see the experiment as life-changing.