Speaker here. Nothing particular although ObjC lacks any language based features to control inheritance. The concern is that if a method has a particular effect

Speaker here. Nothing particular although ObjC lacks any language based features to control inheritance. The concern is that if a method has a particular effect on state (possibly even private internal state) that you expect to take place that subclasses may override the method in such a way that it changes all the expectations and may leave you in an inconsistent state. This means that it is necessary to document (or have conventions for) what should and should not be overridden and whether overriding methods should call the superclass implementation too. In unsafe languages with raw pointers the issues may be more severe than languages like Python and Ruby but they are generally similar. I may have overstated the point too, I had notes but not a script. The point is that you need to be careful designing and documenting for subclassing.