What's the stupidest thing anyone has ever told you as fact?

There are 7 alien spaceships orbiting earth right now the size of Jupiter, but they are invisible. They are not revealing themselves to us yet because we are not ready. But they are mind controlling the world leaders to prevent us from killing each other in a nuclear war. They helped trump win so as to save us all. There are also millions of people living underground in a whole separate society. Hilary Clinton is a lizard person, etc.

Then after he told me all this he said seriously with a straight face. “I will never tell you anything that I don’t know to be true 100%”

Ah, I see that you have met my wife’s uncle Rudi!
a spaceship the size of Jupiter orbiting earth... this guy needs to learn high school physics
Did they learn about it in History Channel?
Oh lord that sounds like serious MH level
What the fuck?

Briefly being around someone with the covid vaccine can mess up your periods and cause early menopause.

Someone actually said that to you in seriousness?
Well, better don't tell them about barcodes unless you want to be responsible for them spending a 1000€ on a decent Hildegard-Orgon-Accumulator...
- 2BG
Oh boy I just had flashbacks of my family sharing so many COVID conspiracy theories during that time. It was so so bad.
- Yep, scary times

The first ones I could think out of The top of my head:

Wind turbines are being powered by Diesel Generators
In The Middle Age people were like 4m tall, which is the reason why the entrance gates of castles are so big.

There are probably way more, that I forgot (my parents are quite deep into conspiracy theories and so is their entire social circle).

In The Middle Age people were like 4m tall, which is the reason why the entrance gates of castles are so big.
This guy clearly never has been to an actual castle or medieval townhouse and bumped his head on every second door...
And the wind turbine one is so bonkers I can't even imagine someone saying that in a non-joke-context...

I'll stop faster because I'm towing a trailer.

My idiot FIL, a chronic tailgater, the last ever time I opted to be a passenger.

If the trailer has very good brakes set to full power even when nearly empty, it's actually true, but I doubt that's the case…
- I actually asked him that at the time
  Oh, is it a braked trailer?
  No.
- In that exact case yes, but I like to operate on the assumption that my trailer brakes are permanently broken. I've had them fail to operate mid-trip because they're fucking trailer brakes.
- I am pretty sure physics says "no":
  Ff <= µ*Fn
  Fn is proportional to the mass of the trailer, so it is irrelevant for the deceleration (a) if it is empty or not (because Ff = m*a).
  µ is a constant relying on the tire/ground connection. Comparable to the towing car (if you happen to have car tire quality ones on your trailer, which might be doubtful)
  .
  On top of that, typical car trailers use overrun brakes that per definition work by exerting an additional force on the towing vehicle in the direction of movement that is then used to activate the trailer brake.

Flat Earth Theory.

It wasn't my first time hearing about it, but it was my first time meeting someone in person who genuinely believed in it. My coworker and I got a 30 minute lecture on why he was right, presented wholeheartedly.

The amazing thing about round earth is how easy it is to prove it. Just visit both hemispheres and compare the orientation of the moon.

A roommate once told me that cows "have one hole for babies and pooping", i.e., a cloaca, but he didn't know that word. I told him that isn't true, they are typical mammals so they have the full complement of parts.

\ I was sitting at my computer so I pulled up a diagram which he said was wrong. Then he asked to show me and started searching for things like, "cow hole", "cow butthole and birth canal", and "cows with just one hole" before I regained my senses and told him to go away.
\
\ The worst part though was the next day when my girlfriend was using my computer and saw the searches so I had to explain to her the conversation and he came back in to defend himself and tried to look up cow smut AGAIN.

I wonder if they believe it's been the same groundhog for the entirety of the tradition, too.

In high school they taught us that Shakespeare's plays weren't written down, that they were re-constructed by audience goers who remembered the lines and wrote them down.

Absolute bullshit.

That green grapes were grapes that weren’t ripe. Couldn’t believe my ears and had to point out that green/white grapes make white wine and red grapes make red wine, there’s no such thing as red grapes that are green because they’re not ripe.

there’s no such thing as red grapes that are green because they’re not ripe.
Son of winegrower here:
Actually, there are.
But you wouldn't want to eat them... :-)
- But you wouldn’t want to eat them… :-)
  or ferment them. You want the sugar for the fermentation, after all.
  (Some folks in my family grow wine for a living. I genuinely miss them.)
- that’s awesome! i’ve watched a bunch of videos on vineyards and it seems like such an intense lifestyle, especially harvest time.
  wait there are red grapes which are unripe and look green? or white grapes can be green before they’re ripe?
Orange wine made with white grapes would probably blow their mind.
- woooooah
Ask them if green apples are just red apples that aren't ripe, too
- 😂😂 I should!
https://youtu.be/05b4PI3oOvU#t=58s
- woah! the more i know! thanks :)

That Jesus had 20kg stones he'd send Earth in the Doomsday. And that would be the day I'd regret not listening to the person saying it.

...mostly because I asked her why should I take the Bible more seriously than the Gîta, the Quran, or the Poetic Edda.

~25y later I'm still waiting those 20kg stones.

I don't even understand that one... So doomsday comes and all you get is a lousy mid-sized stone?
- Imagine stones falling from the sky, on your head. 144k "chosen ones" are spared, everyone else gets fucked. There's fire, brimstone, and all that folklore. That was roughly her description of doomsday.
  And apparently I'm getting rekt because I don't take this sort of superstition seriously. yawn

My biology teacher in 7th grade during sex-ed:

The anus is a one way road

Fuck her homophobic ass.*

*Not literally. Something tells me she's not into that.

I had an economics teacher in high school who warned us that we’re better off at the higher end of a lower tax bracket than the lower end of a higher one, because a higher bracket being a higher percentage of taxation meant you’d lose so much more money that you’d be holding less in the end until you pass the threshold within that bracket where you get back ahead of the tax. An economics teacher.

this is true, at least for my income tax
If the tax isn't incrementing continuously but in discrete steps, this sounds kinda plausible to me... Also I know of certain hard limits in my country that will force you to suddenly pay some additional kind of taxes or loose certain privileges once you cross them.
This means that earning a single Euro more could lead to loosing hundreds or thousands in effect...
- if it were to make you lose certain benefits that is another story. but just based on tax itself, the higher tax brackets are only applied to the money that you earn beyond the threshold amount. for example, if you have 2 brackets, 10% up to 40k and 20% beyond that, and you earn 60k, you would end up paying 4k tax on the initial 40k and then another 4k on the 20k beyond that. there's no scenario in which you would end up earning less due to getting into a higher bracket

Flat Earth - she even had the nerve to call me stupid for not seeing the obvious truth of it.

I had a health teacher explain hypertension is when your muscles get all stiff and that's why you need to stretch to loosen up.

The Anunnaki were ancient astronauts who visit Earth periodically, like every 10 000 years or something like that, I don't remember the exact frequency, but they are supposed to visit us again some time in the future, when it's their time for a new visit. They are so handsome that we can't resist mating with them and the current human race is a cross between the Anunnaki and the humans of the last time they visited us.
If you travel far enough north, you will find a hidden civilization living under the ice at the North Pole that they don't want us to know about.

The first one sounds like it would make an interesting premise for a sci fi series.
- And the second is the premise of several Christmas movies.
Hey, I recently read about that second one in a Mark Lawrence book.

A mother of three afraid to get prostate cancer, like her own dad.

I had to co-worker to whom I had to explain that women don't have a prostate.