My favorite extra-biblical notion in Catholicism is their weird veneration of Mary. Catholics have this belief that Mary was this divine demi-god that was not only a sinless, flawless, human being, but also stayed a virgin her whole life. Despite the fact that the Catholic Bible literally says that she had other kids with Joseph and that Jesus had brothers.
It's this super bizarre JK Rowling-ization of the Bible that a council unilaterally declared, like, 400 years after Jesus. There's no mention of any of the shit they claim Mary did in the actual Bible yet all Catholics I've ever met vehemently defend these belief.
I went to a Hotel Furniture liquidator for some new furniture. Saw a good looking office chair and they only wanted $20 for it.
Brought that bad boy home and only then did I find out that they had sold me a new Herman Miller Aeron for only $20. Completely insane.