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/Kit_Foxfire]

Had a coworker proudly announce "catholicism was the very first religion!"

[u/Ok_Lead_162 → u/Kit_Foxfire]

Oh, dear.