Inheritance means that a class contains fields from other classes.
For regular serialization, the order of the serialization of these inherited classes makes no difference. But with customized serialization, the order that they are read back is significant. When a class is reading data in, it should be able to rely on the superclasses having already had their data read in. The solution is to just reverse the order of the traversal. It is interesting that this problem has never come up in practice.