This is an example of what can happen when a cactus takes up too much water. Cacti are water-hoarding plants. When water is available, the cactus will soak it up. It doesn’t really know when to stop. The reason many cacti have pleats (ribs) is so they can expand and contract depending on the amount of water available. When it has taken up a lot of water, the pleats are wider apart. As the cactus uses up its stored water over time, the pleats will get closer together and eventually the cactus looks shrunken. During the winter when watering is cut back, a cactus can lose up to 50 percent of its volume. Barrel and Saguaro Cactus seem to be prone to skin splitting in landscapes where water is readily available.
The cactus in the photo is an Echinopsis hybrid offset that I potted in early spring. Since I use a moisture meter, I can’t explain why this particular offshoot spit its skin. It is near the edge of the pot on the side where I water, so it might have received more water than the others in the pot. Cacti repair themselves by forming a callus, or scar, and as long as it doesn’t get an infection, the damage is only cosmetic. As the cactus matures, the split area will be less and less noticeable.