I uncovered this one while working on the interop scenario for WS-Eventing. The spec defines two delivery formats, wrapped and unwrapped, and allows for the extensible definition of other formats. However, nothing in the spec says whether or not an Event Source or an Event Sink MAY/SHOULD/MUST implement either of these delivery formats. This creates a potential interop problem if a number of implementations support unwrapped but not wrapped and another set of implementations supports wrapped but not unwrapped (there is also a hole in which an impl may not choose to support either format - nothing in the spec says it has to - and instead only supports its own extension format).