Some examples that came to my mind were the "élan vital" (the belief that life is some sort of substance), the theory of the four humors (one consequence of which was the wide spread use of bloodletting as medical treatment for all sorts of purposes), the static universe, and the non-acceptance of continental drift. On the more absurd side of things is the belief that semen is produced in the brain (because the brain was considered the seat of the soul), and that women who are nursing turn menstruation blood into breast milk. From my recent read of Annie Paul's book "Origins" I further learned that until only some decades ago it was widely believed that pretty much any sort of toxins are blocked by the placenta and do not reach the unborn child. It was indeed recommended that pregnant women drink alcohol, and smoking was not of concern. This dramatically wrong belief was also the reason why thalidomide was handed out without much concerns to pregnant women, with the know well-known disastrous consequences, and why the fetal alcohol syndrome is a fairly recent diagnosis.
I was collecting more examples, not very actively I have to admit, but I found yesterday that somebody saved me the work! Richard Thaler, director of the Center for Decision Research at the University of Chicago Graduate School of Business, is working on a book about the topic, and he's asked the Edge-club for input:
"The flat earth and geocentric world are examples of wrong scientific beliefs that were held for long periods. Can you name your favorite example and for extra credit why it was believed to be true?"
You find the replies on this website, which include most of my examples and a few more. One reply that I found very interesting is that by Frank Tipler:
"The false belief that stomach ulcers were caused by stress rather than bacteria. I have some information on this subject that has never been published anywhere. There is a modern Galileo in this story, a scientist convicted of a felony in criminal court in the 1960's because he thought that bacteria caused ulcers."
I hadn't known about the "modern Galileo," is anybody aware of the details? Eric Weinstein adds the tau-theta puzzle, and Rupert Sheldrake suggests "With the advent of quantum theory, indeterminacy rendered the belief in determinism untenable," though I would argue that this issue isn't settled, and maybe never will be settled.
Do you know more examples?