For example: for 30 years we had research blasting cholesterol and saturated fats as evil, because predecessors had that result, some arguments were completely circular (saturated fats are bad because they raise blood cholesterol, and blood cholesterol is bad because a bad thing raises it... with no research trying to find how much any of the two really affect health).

Another example is thyroid diagnosis, after discovery of TSH test, medics started to use it exclusively, and accuse patients with normal results but with severe and advanced symptoms of having psychological issues, also many thyroid doctors never bother in ever asking for other relevant tests, like t3 and antibodies, and the TSH tests frequently use outright dated target ranges, with medics skeptical of new research because the old way is mostly working in their experience (since by their definition, people with symptoms but normal values aren't sick, thus their treatment is 100% effective, since it helps all sick people, and the "non-sick" are delusional and thus it is failure of other medical fields that they are sick)

Other example is any medic that saw patients improve under certain treatment, and then start to always use that treatment in a cargo cult manner, without paying attention to any research that proves that it is misguided, like all those kids forced to take ADHD medicine in their early life.