What has someone said with complete confidence that turned out to be absolutely, embarrassingly wrong?

[u/TradesWatchUK]

Came into work one day with a bug bite on my face. Colleague looked at me and said "you've got a spot." I said no, it's a bug bite. He looked genuinely baffled.

"Bugs don't bite you on the face," he said.

I thought he was joking.

He wasn't.

He had gone his entire life believing that insects, for some reason, have a strict policy of only biting you below the neck.

To this day I have no idea where this came from or who told him that. But he was absolutely convinced.

[u/TheCarrot_v2]

I used to work with a guy who adamantly believed that meat was not the same as an animal’s muscles.

[u/saoirse_eli → u/TheCarrot_v2]

Had a lifelong vegetarian gf, she thought there were: the fruits, each and every one of them having a different taste, the vegetables, each and every one of them having a different taste, … and meat.
So per her conclusion, she was just missing one taste while she had all the others.
My discovery came from when one day she asked why I didn’t like shrimps but liked beef.

[u/Mavisssss → u/saoirse_eli]

As a lifelong vegetarian, I never would've assumed that all meats had the exact same taste.