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.
{
    LOCK(cs_LastBlockFile);
    CDiskBlockPos posOld(nLastBlockFile, 0);
    FILE *fileOld = OpenBlockFile(posOld);
    if (fileOld) {
        if (fFinalize)
            TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nSize);
        FileCommit(fileOld);
        fclose(fileOld);
    }
    fileOld = OpenUndoFile(posOld);
    if (fileOld) {
        if (fFinalize)
            TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nUndoSize);
        FileCommit(fileOld);
        fclose(fileOld);
    }
}
static bool FindUndoPos(CValidationState &state, int nFile, CDiskBlockPos &pos, unsigned int nAddSize);
static bool WriteUndoDataForBlock(const CBlockUndo& blockundo, CValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)