And extremely common one in my cultural (USA) context is that "being cold" can give you a "cold". According to common wisdom, being out in cold weather with wet hair and no hat will cause you to be infected with a rhinovirus.
The expression encapsulating this is "You'll catch your death!"
When travelling in Eastern Europe, I found a similar attitude towards drafts from a couple locals. The safest was to keep all windows and doors to the outside closed (in a room or car). Opening one portal was frowned on. Two or more (creating airflow) was taboo and would cause sickness.
Prolonged exposure to cold temperatures can weaken the immune system, so the advice is at least helpful, though false.
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?