The following can happen:
A client is developed against a level B extension "X". It connects to a server that has been developed against another level B extension "Y". In the case where extensions X and Y both contain a level B extensions class (i.e. a class with a tagged value named "extension" and set to "levelb") with the same name, the client will misinterprete the extension content with unpredictible result.
Add a unique element (e.g. UUID) to the name of levelB extensions classes, i.e. all classes with a tagged value named "extension" and set to "levelb".
No longer an issue since the introduction of namespaces.