The Rosenhan Experiments
In 1973, psychologist David Rosenhan and several of his colleagues presented themselves at psychiatric hospitals around the USA feigning hallucinations. All of the group were admitted and diagnosed with mental disorders, after the initial diagnosis each of the "patients" claimed to experience no further symptoms, some were released after 7 days, some were kept in the institutions for rather longer. Rosenham himself was confined for almost two months and was only released when he "admitted" to having been ill.
In a second experiment, done this time with the knowledge of a psychiatric hospital, Rosenham told the hospital he would be sending them a number of psuedo-patients and challenged them to detect the false patients from those who were actually ill. Doctors and staff at the hospital declared that they had spotted 41 psuedo-patients, whereupon Rosenhan told them he had actually sent none at all.
Rosenham's experiments caused outrage in the psychiatric profession, but later experiments on confirmation bias by Temerlin and Loring & Powell would produce similar results.