When objects were added in savepoints and either the savepoint was
  rolled back (https://bugs.launchpad.net/zodb/+bug/143560) or the
  transaction was aborted
  (https://mail.zope.org/pipermail/zodb-dev/2010-June/013488.html)
  The objects' _p_oid and _p_jar variables weren't cleared, leading to
  surprizing errors.