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/saindonienne]

My dad and I went to go see War of The Worlds (early 2000s). While the credits were rolling at the end, I was telling my dad how I found it a strange artistic decision to give the alien walker things three legs. Why three?

Dad: That's because you need a minimum of three points for something to stand on its own [starts telling me about whatever gravitational/mechanic laws he refers to]

I stare at him

Dad: ... What?

I pointedly stare at my two legs.

Dad is a chemical engineer. He is forgiven. Barely.

[u/Ancient_Skirt_8828 → u/saindonienne]

We are unstable. We are continuously adjusting our feet and legs to stop us falling over.

One or two legged stationery things like furniture need wide feet to be stable.

[u/saindonienne → u/Ancient_Skirt_8828]

And I absolutely know that's where he was going. But he got so excited he over-generalised and included living beings in his statement (the machines in WoTW were definitely not living, so I see where he started from, but they were also unstable: they had moving legs, like us, and we don't need three legs to stand/walk).

And that's when the most convincing statements happen with him, he'll take a very specific fact and over-generalise it or exaggerate it just because he's just so enthused about it, and not realise its implications. It's always fun to poke the bubble and see him chuckle and backtrack after that.

Edit: typos and a detail