Class is permanent

A tale to warm the hearts of us old fogeys everywhere. The great Dutch veteran GM, who nowadays plays relatively little, reaches what should be a drawn rook ending, against a talented, much younger player, and then gives a master class. White's vastly more active king means that Black is always suffering in the ending, but L'Ami admitted today on her Twitter feed that:

Had a draw yesterday in a rook endgame but Timman showed once again his class...I lost the game but learned a lot!

Can you work out exactly where Black went wrong?