Comment 55 for bug 232364

To quote Cody in this very thread:

"I realized that it only occurs when something tries to access dbus and there is no session bus already started. A quick check showed that gnome-screensaver which is launched just shortly before dbus --sh-syntax --exit-with-session depends on dbus which lead me to hypothesis that the race is between the two dbus-launch processes caused by the libxcb issues."

There is your answer.