When UML 2.4 was released we discovered an issue that meant it was impossible to interchange StructuredActivityNodes reliably. StructuredActivityNodes are executable nodes in an activity diagram that are also groups.  UML 2.4 doesn’t specify clearly whether they should be serialized as nodes or as groups; as a consequence, different tools do different things.  In UML 2.4.1 this ambiguity is remedied by serializing them in their own collection. UML 2.4.1 can be found at http://www.omg.org/spec/UML/2.4.1/ and all of the machine-readable files are at http://www.omg.org/spec/UML/20110701/.

This is the first time in the history of UML that a complete machine-readable definition of the language is available online, defined as an instance of itself, and with no dangling references.  Only of interest to UML aficionados maybe, but a milestone nonetheless.