People taking theories as cannon

Have you ever encountered people arguing a theory as a fact to counter someone else point?

What theories do you know to be taken as a fact for the franchise even though it is never openly explained?

For me is the detailed account on the war before OoT. Especially on the Sheikah and the royal family. There is a theory going with insane detail I feel gets treated like cannon.

I think many also take game theory a bit too much like a fact at times. What do you guys, think?