When pressing the red close button in the window border the GTK application reliably crashes. It doesn't seem to matter if it'sa toplevel window, or an about dialog. Red button = instant death.

Test application:
public class CrashingWindow
{
	public static void main(string[] args)
	{
		Gtk.init(ref args);

		var window = new Gtk.Window();
		window.show();

		Gtk.main();
	}
}

Debug details:
http://30a2d709d7791c22.paste.se/

Specific GTK version: 3.6.1

If an application instead for some reason call the g_application_quit function, it will shutdown properly without a crash.