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.
CValidationState state;
    const CChainParams& chainparams = Params();
    FlushStateToDisk(chainparams, state, FlushStateMode::ALWAYS);
}
void PruneAndFlush() {
    CValidationState state;
    fCheckForPruning = true;
    const CChainParams& chainparams = Params();
    FlushStateToDisk(chainparams, state, FlushStateMode::NONE);
}
static void DoWarning(const std::string& strWarning)
{
    static bool fWarned = false;
    SetMiscWarning(strWarning);
    if (!fWarned) {
        AlertNotify(strWarning);
        fWarned = true;
    }
}
void static UpdateTip(const CBlockIndex *pindexNew, const CChainParams& chainParams) {