It's only the king that can't pass through a threatened square. The rook can :)
Bug: black castle when he shouldn't be able to
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Ooooh, thanks SiggyYo :) My bad then.
That's a legal castle. The rule is that the king may not move through check while castling, the castle can move through a contessted square if it wants to though.
just be happy he didnt play Bxb2 
Hello,
In the following game http://www.chess.com/livechess/game?id=807578568 black was able to castle long after 12. Bf4, but this black bishop move was looking at b8 right before he castled... so he shouldn't have been able to castle.
Except if I don't remember the rules of castling correctly, I think this is a bug. I thought that you couldn't castle if the King, Rook or any square in between the castle path were attacked.
What's annoying is that I *intentionally* moved my bishop there to prevent castle... and then he castled anyway :)