The one that I can think of is cracking your knuckles, which supposedly causes arthritis. It's beyond me how cavitation can cause an autoimmune disease, and seems much more likely to me that people are bothered by the sound and settled on a joint ailment as a deterrent. (Studies have shown there is no link, but that cracking your knuckles is correlated with other hand problems. I suspect that's because people with those problems are more likely to be able to / get relief from cracking their knuckles.)
It's beyond me how cavitation can cause an autoimmune disease, and seems much more likely to me that people are bothered by the sound and settled on a joint ailment as a deterrent.
Osteoarthritis, which is the most common form of arthritis, is not an autoimmune disease; rather it seems to be related to aging, including cumulative wear and tear on joints. (Rheumatoid arthritis is an autoimmune disease.) Cavitation can mess up hydraulics components pretty bad, so I do think this passes the common-sense test. Apparently it doesn't pass the evidence test, though.
Today I received some shoes in the post, which included a couple of packets of silica gel. I don't think I have ever seen a packet of silica gel that didn't have "DO NOT EAT" printed on it, and it's always bemused me. It doesn't look edible, or smell appetising, and isn't even especially harmful to ingest in most circumstances. Chances are that if I ever did want to eat silica gel, I'd probably have a damn good reason, and a lifetime of being told to not eat it is an obstacle to that.
This has started me thinking about all the other things we internalise as serious hazards contrary to reality. As a child, I was told that picking my nose and eating it would have some sort of cumulative toxic effect. This was obviously a lie manufactured by my parents (or maybe their parents) to get me to stop doing it, but a couple of decades later I felt positively scandalised when I read about an Austrian pathologist who claimed the practise was beneficial to the immune system. (Although this is mentioned in the delightful Wikipedia page on nose-picking, the reference links are dead, so I'd actually treat this assertion with caution, but feel free to munch away on your own dried nasal mucus anyway).
Although nose-picking and eating the packaging that shoes come in are pretty trivial examples, I do wonder how many of these prohibitive false dire consequences I'm still labouring under, invisibly making my life more difficult. I also wonder how many full-grown adults still don't accept sweets from strangers.
Do you have any examples of an authority figure, or a prevailing piece of cultural conditioning, giving warnings of dire outcomes you later discovered to be false, misleading or based on an agenda you were naive to at the time?