extreme watercore in apple (image : Red58bill)
This is appearing in the disease section, but does not really fit within this category. It is however often thought of as such. Really an affliction of poor management, but with some varieties being far more prone than others. The above photo is an extreme example, most affected apples show only a small degree of damage. It is usually a  sign of boron/calcium imbalance. The sugars are concentrated in the area, and such apples are sometimes sought, but normally is considered less than desirable.