In the early days of human dissection and anatomy, bodies were often in short supply. A whole underground of cadaver selling developed, whether the bodies were obtained legally or not was not much of a concern. Some dug up newly buried bodies, others were obtained by some very nefarious practices.
"Burking" Named for William Burke who lured beggars into his home and strangled them. He then sold the bodies to anatomists.
One very common fact was that most if not all bodies studied were poor. Being poor brought other health issues that were unknown at the time. Essentially the had lived a life of chronic stress. Malnourished, worried about being killed, etc... This chronic stress made them produce more stress hormones. The increase in stress hormones caused their adrenal glands to enlarge. When physicians studied these cadavers an enlarged adrenal gland was considered"normal."
When a wealthy man came into the morgue or anatomy room they presented with an "undersized" adrenal gland. They made up a condition called Idiopathic adrenal atrophy. This disease was wide spread in early twentieth century. Years later they realized they were mistaken about the size of the adrenal gland and then everyone was cured.
A more serious error came in the 1930's with the Thymus gland. Still unknown to the medical community that stress can atrophy organs, lead to some very bad outcomes. Babies suddenly dying in their sleep were being studied, Sudden Infant Death Syndrome. (SIDS). We presently think now that it occurs when babies in the 3rd trimester receive less oxygen then normal and brain cells are damaged that help control respiration. But, back then, they had no clue why babies were suddenly dying.
At the end of the 19th century a pathologist autopsied SIDS babies and non SID babies. What he found was that SIDS babies had a much larger thymus gland. Now we know that the non SID babies had died of chronic stressful illness that had atrophied the thymus gland. They drew the conclusion that in SIDS, the "abnormal" large thymus gland was pressing down on the trachea causing suffocation.
By 1920 this condition had a name and it was in all the leading pediatric textbooks. To prevent this the thymus should be irradiated to shrink it. This advice persisted into the 1950's. Obviously, this had no effect on SIDS, but what it did do is irradiate the gland next to it, the Thyroid gland. This eventually lead to thousand and thousands of cases of thyroid cancer.
As you can see, mistaking normal can have tremendous consequences. Every now and then it would be interesting, if not good practice, to reconsider what normal is to you and think about the opposite or how you could look at normal a little differently.
"What mistake are we making now, in our modern scientific ignorance, and how many people will ultimately pay for it."