From Stephan Huber, "just a follow-up: I looked at the code of osg/Referenced.cpp and notice

a discrepancy in getOrCreateObserverSet. For the atomic- and
mutex-codepaths the newly created observerset gets manually ref'ed, but
not in the codepath used when OPENTHREADS_ATOMIC_USE_MUTEX is defined. I
added the manual ref and tadaaa, the crash went away."

src/osg/Referenced.cpp

@@ -282,7 +282,11 @@ ObserverSet* Referenced::getOrCreateObserverSet() const
     }
     else
     {
-        if (!_observerSet) _observerSet = new ObserverSet(this);
+        if (!_observerSet)
+        {
+            _observerSet = new ObserverSet(this);
+            static_cast<ObserverSet*>(_observerSet)->ref();
+        }
         return static_cast<ObserverSet*>(_observerSet);
     }
 #endif