Without semantic knowledge, it is not always possible to know in advance whether a shallow copy of the object is sufficient, whether a deep copy is required, or whether a correct copy requires something in between the two.

A shallow copy of an object treats instance data as static data that can be simply copied into a new instance. For some objects, including those based on arrays or hashes where all values are simple scalars, this works. But as soon as a value is a reference to another object, you have to make a choice between simply copying the reference (i.e., making a shallow copy), or recursively cloning the referred-to object (i.e., making a deep copy). In a moderately sophisticated system, you're liable to run into objects that require some combination of shallow and deep copying in order to be successfully cloned.