The other day we had a problem whereby we had a parent and child object and the implementation of the child's hashcode method used the parents hashcode + the childs hashcode

well the problem lie in the fact that when the parent and child were eagerly loaded and then later the parents state was changed and some code that wanted to find the children in the parents linked hashset couldn't because the hashcode when the lookup was performed had changed from when the data was originally fetched and put in the map.

So I guess the lesson is that you shouldn't tie the child and parent's hashcode's if you're expecting to lookup the children in map after the parents state has changed such that it produces a different hashcode value.

The hashcode should never change during the lifetime of an object!
More about how to implement hahcode: http://docs.jboss.org/hibernate/stable/core/reference/en/html_single/#persistent-classes-equalshashcode