I've been playing chess for more than a year, only to discover recently that you CAN castle when you're a square that your rook passes is attacked! (queenside castle when b1 or b8 is attacked) Although chess.com's diagram editor doesn't allow me to post a diagram showing that. Kinda weird, don't ya think?

It's logical, there are rules against putting the king in danger, none about putting rooks in danger.

You can castle when the rook is being attacked as well.

Omg, I just realised, you're right!
From fide.com:

"(2) Castling is prevented temporarily

1. if the square on which the king stands, or the square which it must cross, or the square which it is to occupy, is attacked by one or more of the opponent`s pieces.

2. if there is any piece between the king and the rook with which castling is to be effected."

Says nothing about the rook. And all this time I though that the rook cannot be attacked, not can it pass an attacked square while castling :O

sounds very odd. are you sure that only the rook is under attack? if this is the case there is a bug in the chess.com diagram.

In his Candidates with Karpov in 1974 Korchnoi got up and asked the arbiter if he could castle with his rook under attack. Assured that he could, he did, and soon won. That's an odd one isn't it.

You cannot castle through a checked square, but that definitely only applies to the king.

Anyway, seems like chess.com's games have errrorsrrr...

I know of that episode too, though the first time I heard it I wondered if his asking was just a psychological ploy. His moves leading up to that point certainly included that 0-0 in the calculations.