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.
#include <script/sigcache.h>
#include <script/standard.h>
#include <timedata.h>
#include <tinyformat.h>
#include <txdb.h>
#include <txmempool.h>
#include <ui_interface.h>
#include <undo.h>
#include <util.h>
#include <utilmoneystr.h>
#include <utilstrencodings.h>
#include <validationinterface.h>
#include <warnings.h>
#include <future>
#include <sstream>
#include <boost/algorithm/string/replace.hpp>
#include <boost/algorithm/string/join.hpp>
#include <boost/thread.hpp>
#if defined(NDEBUG)
# error "Bitcoin cannot be compiled without assertions."
#endif
#define MICRO 0.000001
#define MILLI 0.001
/**
 * Global state
 */
namespace {
    struct CBlockIndexWorkComparator
    {
        bool operator()(const CBlockIndex *pa, const CBlockIndex *pb) const {