If the type of an element "a" is a union and another element "b" specifies "a" as its substitution group affiliation and its type is or is derived from one of the member types of the union, Xerces will report an error if an instance attempts to substitute "b" for "a".

<xs:element name="a" type="myUnion"/>
<xs:element name="b" substitutionGroup="a" type="xs:double"/>
<xs:simpleType name="myUnion">
 <xs:union memberTypes="xs:double xs:dateTime"/>
</xs:simpleType>

This bug is causing 2 JCK test to fail:
xml_schema/msData/particles/jaxb/particlesZ012.html#particlesZ012.v
xml_schema/msData/particles/jaxp/particlesZ012.html#particlesZ012