From Robert Osfield and Richard Schmidt, made fixes inspired by an original submission from Richard which

highlighted problems with Light, ClipPlane and Hint usage in osg::State's usage of cloneType
and reassignment of target/num in StateSet/these StateAttributes.

src/osg/ClipPlane.cpp

@@ -11,6 +11,7 @@
  * OpenSceneGraph Public License for more details.
 */
 #include <osg/ClipPlane>
+#include <osg/StateSet>
 #include <osg/Notify>
 
 using namespace osg;
@@ -28,7 +29,43 @@ ClipPlane::~ClipPlane()
 
 void ClipPlane::setClipPlaneNum(unsigned int num)
 {
+    if (_clipPlaneNum==num) return;
+
+    if (_parents.empty())
+    {
+        _clipPlaneNum = num;
+        return;
+    }
+
+    // take a reference to this clip plane to prevent it from going out of scope
+    // when we remove it temporarily from its parents.
+    osg::ref_ptr<ClipPlane> clipPlaneRef = this;
+
+    // copy the parents as they _parents list will be changed by the subsequent removeAttributes.
+    ParentList parents = _parents;
+
+    // remove this attribute from its parents as its position is being changed
+    // and would no longer be valid.
+    ParentList::iterator itr;
+    for(itr = parents.begin();
+        itr != parents.end();
+        ++itr)
+    {
+        osg::StateSet* stateset = *itr;
+        stateset->removeAttribute(this);
+    }
+    
+    // assign the clip plane number
     _clipPlaneNum = num;
+
+    // add this attribute back into its original parents with its new position
+    for(itr = parents.begin();
+        itr != parents.end();
+        ++itr)
+    {
+        osg::StateSet* stateset = *itr;
+        stateset->setAttribute(this);
+    }
 }
 
 unsigned int ClipPlane::getClipPlaneNum() const