Hi have problem that appears at random: I store Persistent Objects (inherited from a Zope Folder) as keys in a OOBTree. I developed something like mxmRelation (http://www.zope.org/Members/maxm/productList/mxmRelations) It is strange: I get "r" from the keys of the hash, but if I try to get it from the hash it fails. This only happens sometimes. def tostring(self): "return string of the relations" result={} for r in self._tree.keys(): result[r]=[] hash_back=self._tree.get(r, None) if hash_back==None: print "tostring(): ", r raise Exception, "Internal Error ???" return for r2 in hash_back.keys(): result[r].append(r2) return result It is a single threaded application I use the version for win32: Zope 2.5.0 (binary release, python 2.1, win32-x86), python 2.1.2, win32