If damaged, normally green and succulent Saguaros (Carnegiea gigantea) can grow thick, hard, gray, callused scar tissue over injuries. I took this photograph of extensive scar tissue on a living Saguaro along Catalina Highway.
I don't know what caused this damage to the Saguaro, perhaps it was merely accumulated injuries over its long lifetime. One thing that can damage Saguaros is extreme cold. Saguaros grow at elevations below 3600 feet (1097 m) here in Arizona because they are cold-sensitive.
Fire can also damage Saguaros, and they are very susceptible to it. Large fires used to be practically unheard of in the desert areas where Saguaros grow because of the low fuel supply (few trees, lots of cactus and bare dirt or rocks), but this has changed since human-introduced, non-native weeds and grasses have invaded these desert areas. When these invaders dry out in late spring, they unfortunately provide ready fuel for fires. Smaller Saguaros are killed outright by fire, but larger Saguaros can survive it, or at least linger on with the fire damage for a few years.