This isn't about how the young people at work don't know what the capital of Spain is, or that the petrol indicator on the car dashboard tells you what side your petrol cap is on, etc. More things that you didn't know at one point, but having learned about it (or more likely - experienced it yourself) can't see why it isn't more widely known.
My examples:
Diagnosing of gallstones. My GP failed to diagnose my dreadful gallstone pain, in the end (after suffering for years) I asked a forum like this about my symptoms and loads of people said it's probably gallstones. I went to a different GP and said 'I think it could be gallstones' and it was - got the gallbladder removed and not had a problem since.
Anyway, since then I realise the symptoms are fairly obvious (I know it's hard to describe pain but from my own experience I know what sort of questions to ask) yet there are always threads on here about how gallstones didn't get suggested by a doctor/got missed, a friend of mine also had similar experience just not knowing what this indescribable pain was. How on earth do doctors not know to suggest/check for gallstones? It seems really common?
Other misconceptions:
Waters going just before you're about to give birth (most people who've had children probably know this isn't accurate! My waters had to be broken when I was in labour!) This still happens on TV all the time!
Schizophrenia = multiple personality disorder. It's not.
Breastfeeding - so much I had no idea about! How painful it can be to start with. That babies need feeding all the time! I did an NCT breastfeeding class and I still felt that it was based on a lot of assumed knowledge (that I didn't have!). Even calling formula feeding 'bottle-feeding' but it not actually being about feeding from a bottle but the fact it's formula not breastmilk.
Colic - that it's a medical problem or indigestion, reflux etc. It's hard to tell from a lot of the stuff that's written about it but it just means lots of unexplained crying (usually at certain times etc, well it was in my experience). I actually found that 'purple crying' was a better explanation: purplecrying.info/what-is-the-period-of-purple-crying.php
Bit more obscure but d-mer.org/ - D-MER - very specific feeling of sadness when breastfeeding. I wish I'd known about this with my first baby!
Another example might be anyone that still thinks 'common-law wife' is a thing. Although I don't see many people using that phrase apart from to point out it's not a thing (legally etc)!