code: private T verifyNewInstance(T t) {
          if (t!=null && t.getDescriptor()!=this) {
              // TODO: should this be a fatal error?
              LOGGER.warning("Father of "+ t+" and its getDescriptor() points to two different instances. Probably malplaced @Extension. See http://hudson.361315.n4.nabble.com/Help-Hint-needed-Post-build-action-doesn-t-stay-activated-td2308833.html");
          }
          return t;
      }
docs: Look out for a typical error a plugin developer makes.
  See http://hudson.361315.n4.nabble.com/Help-Hint-needed-Post-build-action-doesn-t-stay-activated-td2308833.html