The trivial example demonstrating the crash is up at:
https://bitbucket.org/imonroe/emoji_addapplicationfontcrash

The code is really simple:

    QApplication app( argc, argv );
    QFontDatabase::addApplicationFont("../Symbola.ttf");
    QLabel window;
//snip: some irrelevant code
    window.setText("Cyclone <br> /*emoji char here, JIRA doesn't like it*/");
    window.show();
    return app.exec();

So here I add the Symbola font, which contains emoji characters. Then after trying to display an Emoji character it crashes in QTextLayout.

It happens 100% of the time on my home computer and never on my work computer. They both run openSUSE 12.3, use the nvidia X11 driver, and I've tested with the same build of Qt. The work computer is faster so maybe it is a race condition? Or due to different fonts installed at home?

A coworker reported reproducing the issue on his Ubuntu computer.