Tuskegee syphilis experiment
Researchers started using tricks and lying to the men in the study, without caring about medical ethics or the men's rights.  For example, researchers wanted to do lumbar punctures ("spinal taps") on the men to measure the effects of syphilis.  These spinal taps were dangerous and very painful.  To make sure that the men would come get the spinal taps, the researchers sent all 400 subjects a letter titled "Last Chance for Special Free Treatment."  This was a lie; the spinal taps were not treatment.