 The crown wash at the top of the chimney. All these things, I know there's a lot of things that can go wrong with a chimney. But if you start having some water intrusion into your home, in a lot of cases, if it's on the side of the chimney, that's exactly what it is. That water gets down between the liner and the brick from the crack in the crown wash at the top of the chimney. Nobody can really see what that looks like unless you get up on the roof