I'm going to list a few things that have changed in my lifetime or not so long ago, or "truths" that turn out not to be true, and I invite you to add others. Google away, if you want to. Directions for making a clickable link in a comment field are in the sidebar, but if that's confusing to you just leave the URL and I'll come and enliven a link for you. Comments can be as long and as frequent as you want. Have fun!
Don't Believe Everything you Hear... from doctors.
"Nursing mothers have to drink lots of milk so they can make milk." Cows don't drink milk. People don't need to drink milk to create milk. It's nuts. Most adult humans are lactose intolerant anyway, but it hasn't kept schools and hospitals from giving milk out like crazy, and sometimes insisting that it be finished off.
"Four out of five doctors prefer Camels." (Magazine ads in the 1960's, which also explained how soothing to the throat smoking was, and helpful to the nerves.)
Dr. Dio Lewis, a prominent late-19th Century doctor, was sure that the northern U.S. created strong, wise men. The Carolinas had a climate that emasculated the settlers, and Southern California caused deterioration, loss of learning and of interest in ideas, and people who live in southern climates fall into gossiping. The exact quote is here.
Don't Believe Everything you Hear... from teachers.
"Your permanent record will follow you forever."
When I went to teach in the school district I had attended for ten years, I asked to see my permanent record. It was legal to make the request, and if it was going to follow me everywhere, why couldn't it be produced for a few minutes for a school employee to see? Uh.... "They're in storage and it would take a while to find it." I was like 22 years old and it had already quit following me!? I just laughed; I didn't press it. I'm just as glad not to know what insipid things were in there. The principal had written on my teacher review that I had "a good rapore with students."
"Brush your teeth up and down," which was replaced in a few years by "Brush down on the top teeth, and up on the bottom teeth," which was replaced within about a year by "Make the toothbrush go in circles." That might could go under don't believe everything you hear from doctors, but I learned it from teachers in health classes, telling us what the dental profession had learned to be crucial, tooth-saving Truth.
A friend of ours is an EMT and says mouth-to-mouth resusitation is not considered a good thing now. Lots of us who had red-cross cards over the years were told otherwise. Advice has changed on treatment of burns and on tourniquets, too. I put that under teachers instead of doctors because I learned first aid in school at in Girl Scouts. Here's one article on the changing stance, and many more can be found with a web search.
Don't Believe Everything you Hear... from parents.
"If you stick your tongue out your face will freeze like that."
Existing collection about parenting: "If I let him, he would..."
"Masturbation will [do various specific and unfounded things to] you." [Because of masturbation, "People would lose flesh, they would get weak, they would cough, and they would end up with tuberculosis, which of course he called consumption. " Read more about that here, including the original purpose of Kellogg's cornflakes. Eeyew. (And this, too, could've gone under the doctors' list but most people heard it from their parents and this might help explain why.)
"If kids play with guns they'll become violent." SandraDodd.com/peace/guns
Some of the best links and examples left in comments below might be added above. Expect the main entry to change, is what I mean to say. —Sandra