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.
return OpenDiskFile(pos, "blk", fReadOnly);
}
static FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly) {
    return OpenDiskFile(pos, "rev", fReadOnly);
}
fs::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix)
{
    return GetBlocksDir() / strprintf("%s%05u.dat", prefix, pos.nFile);
}
CBlockIndex * CChainState::InsertBlockIndex(const uint256& hash)
{
    AssertLockHeld(cs_main);
    if (hash.IsNull())
        return nullptr;
    
    BlockMap::iterator mi = mapBlockIndex.find(hash);
    if (mi != mapBlockIndex.end())
        return (*mi).second;
    
    CBlockIndex* pindexNew = new CBlockIndex();
    mi = mapBlockIndex.insert(std::make_pair(hash, pindexNew)).first;
    pindexNew->phashBlock = &((*mi).first);
    return pindexNew;
}
bool CChainState::LoadBlockIndex(const Consensus::Params& consensus_params, CBlockTreeDB& blocktree)