Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
};
static CMainSignals g_signals;
void CMainSignals::RegisterBackgroundSignalScheduler(CScheduler& scheduler) {
    assert(!m_internals);
    m_internals.reset(new MainSignalsInstance(&scheduler));
}
void CMainSignals::UnregisterBackgroundSignalScheduler() {
    m_internals.reset(nullptr);
}
void CMainSignals::FlushBackgroundCallbacks() {
    if (m_internals) {
        m_internals->m_schedulerClient.EmptyQueue();
    }
}
size_t CMainSignals::CallbacksPending() {