For example, given:

<class Order ...>
    <composite-id>
        <key-property name="customerId" .../>
        <key-property name="orderNumber" .../>
    </composite-id>
</class>

the following calls all fail to locate the property:
cfg.getClassMapping( Order.class.getName() ).getProperty( "customerId" );
cfg.getClassMapping( Order.class.getName() ).getReferencedProperty( "customerId" );
cfg.getClassMapping( Order.class.getName() ).getRecursiveProperty( "customerId" );

This is due to the special handling of the identifier...