Steps to reproduce:
The little program at the end of the description reproduces the problem

1. Create an icon larger than 255x255 and save as png
2. Set this icon to be the only icon for the application with
gtk_window_set_default_icon_list
3. Open the main window and see the icon work properly
4. Open a dialog window and the program crashes with an X Window System error.


--- cut here ---
/******************************************************************************
 * Program to test icons
 *
 * 1. Build the program
 * 2. Create several icons with sizes like 8x8, 32x32, 256x256, 1024x1024
 * 3. Run the program
 *      ./progname <iconfile>
 * 4. Press the button to check the icon of the message
 *****************************************************************************/

#include <glib.h>
#include <gtk/gtk.h>

static void dia(void)
{
    GtkWidget   *ad;
    ad = gtk_message_dialog_new (NULL, 0, GTK_MESSAGE_INFO, GTK_BUTTONS_OK,
            "If you can read this message, the icon is fine!");
    gtk_dialog_run(GTK_DIALOG(ad));
    gtk_widget_destroy(ad);
}

gint main (gint argc, gchar *argv[])
{
    gchar       *app_icon = NULL;       /* Name of program */
    GtkWidget   *win;                   /* Main application window */

    gtk_init(&argc, &argv);

    if (argc == 2)
        app_icon = argv[1];

    if (app_icon)
    {
        GdkPixbuf       *pixbuf;        // Pixbuf with icon
        GList           *list;          // List of icons

        pixbuf = gdk_pixbuf_new_from_file(app_icon, NULL);
        if (!pixbuf)
            g_warning("Problems loading icon %s", app_icon);
        else
        {
            list = g_list_append(NULL, pixbuf);
            gtk_window_set_default_icon_list(list);
        }
    }

    win = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(G_OBJECT(win), "delete-event",
            G_CALLBACK(gtk_main_quit), NULL);

    {
        GtkWidget       *but;

        but = gtk_button_new_with_label("Click me (and pray)");
        gtk_container_add(GTK_CONTAINER(win), but);

        g_signal_connect(G_OBJECT(but), "clicked", G_CALLBACK(dia), NULL);
    }

    /* Set the values of some fields */
    gtk_widget_show_all(win);
    gtk_main();
    return 1;
}


Stack trace:
Sorry, didn't manage to get a real stack trace yet...

> ./t 511.png
The program 't' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadLength (poly request too large or internal Xlib length erro'.
  (Details: serial 1358 error_code 16 request_code 18 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)


Other information:
System I'm using is the default FC4 system with updates applied.