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.
{
    uiInterface.NotifyAlertChanged();
    std::string strCmd = gArgs.GetArg("-alertnotify", "");
    if (strCmd.empty()) return;
    
    
    
    std::string singleQuote("'");
    std::string safeStatus = SanitizeString(strMessage);
    safeStatus = singleQuote+safeStatus+singleQuote;
    boost::replace_all(strCmd, "%s", safeStatus);
    std::thread t(runCommand, strCmd);
    t.detach(); 
}
static void CheckForkWarningConditions()
{
    AssertLockHeld(cs_main);
    
    
    if (IsInitialBlockDownload())
        return;