When loading a SWF into a Flash movie you can specify the application domain which the loaded movie will use. If this application domain is a child domain of the loader’s (or other movie’s) domain and “the child defines a class with the same name as a class already defined by the parent, no error results; the child simply inherits the parent’s definition of that class, and the child’s conflicting definition goes unused unless either child or parent calls the ApplicationDomain.getDefinition() method to retrieve it.” (quoted from the documentation)
The first part of the quoted sentence is true, but the second one is not working. Calling getDefinition() on the child domain will return a reference to the parent’s class instead of the child’s original one. I think this is rather a mistake in the documentation than a player bug, since it seems quite logical: referencing a class by the normal way and through getDefinition() should evaluate to the same value. Anyway, I already posted a small note about that to the online documentation.