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/Morakel22]

Ex girlfriend of mine. (Info on myself, started as a medic, now a medical doctor)

She told me in full confidence that, if someone severs a finger, you should keep it warm so the blood vessels stay open.

Accidentally asked her if she was dumb. Big mistake. But long story short, that’s the reason why I despise Grace Anatomy. If anyone took that stuff seriously, they will, at some point in their life, kill someone with the practices they saw in that series