It may happen so that disconnect() is called in s_joined state just an

instance before the SYNCED event is delivered. Thus wsrep-lib is already
in s_disconnecting state but still will transition to s_synced albeit
with a warning. This is pointless and possibly harmful.
If we are disconnecting ignore any remaining state events from the
replication queue.
Refs codership/wsrep-lib#251

src/server_state.cpp

@@ -1364,6 +1364,10 @@ void wsrep::server_state::state(
 
     if (allowed[state_][state] == false)
     {
+        /* If we are disconnecting ignore any remaining state events from
+         * the replication queue */
+        if (s_disconnecting == state_) return;
+
         std::ostringstream os;
         os << "server: " << name_ << " unallowed state transition: "
            << wsrep::to_string(state_) << " -> " << wsrep::to_string(state);