the bug has been opened on https://bugs.launchpad.net/ubuntu/+source/gnome-settings-daemon/+bug/246447

"This error occasionally appears (mostly on login, but not always, like this report), but I can't work out the trigger. My computer had been sitting idle for 7 hours with Transmission using eth0, then I started rhythmbox playing, and opened both firefox and thunderbird. This would be the point that the crash occurred. I noticed compiz greying out the rhythmbox window so there was unusually high cpu activity occuring during this crash (but perhaps that was just apport).

This looks similar to some of the other active gnome-settings-daemon bugs, but this is the only one that crashed in g_slice_alloc().

http://launchpadlibrarian.net/16481965/valgrind.log
    *   valgrind.log  (103.1 KiB, text/plain)

==13835== Invalid read of size 1
==13835==    at 0x6F57EF0: SendSetMap (XKBSetMap.c:331)
==13835==    by 0x6F58B0B: XkbSetMap (XKBSetMap.c:532)
==13835==    by 0x12761783: XkbWriteToServer (srvmisc.c:52)
==13835==    by 0x12549C9A: xkl_config_get_keyboard (xklavier_config_xkb.c:300)
==13835==    by 0x1254A347: xkl_xkb_activate_config_rec (xklavier_config_xkb.c:471)
==13835==    by 0x12338AB3: gkbd_keyboard_config_activate (in /usr/lib/libgnomekbd.so.2.0.0)
==13835==    by 0x11F2D060: apply_xkb_settings (gsd-keyboard-xkb.c:187)
==13835==    by 0x11F2C8C0: gsd_keyboard_manager_start (gsd-keyboard-manager.c:402)
==13835==    by 0x11F2C1E6: impl_activate (gsd-keyboard-plugin.c:78)
==13835==    by 0x405471: gnome_settings_plugin_info_activate (gnome-settings-plugin-info.c:506)
==13835==    by 0x404094: maybe_activate_plugin (gnome-settings-manager.c:93)
==13835==    by 0x99F8C8C: g_slist_foreach (in /usr/lib/libglib-2.0.so.0.1704.0)
==13835==    by 0x404271: gnome_settings_manager_start (gnome-settings-manager.c:271)
==13835==    by 0x403E91: main (main.c:312)
==13835==  Address 0x129dbcb8 is 0 bytes after a block of size 256 alloc'd
==13835==    at 0x4C24384: calloc (vg_replace_malloc.c:397)
==13835==    by 0x6F63008: XkbAllocServerMap (XKBMAlloc.c:170)
==13835==    by 0x127721B9: ReadXkmSymbols (xkmread.c:647)
==13835==    by 0x12772C3A: XkmReadFile (xkmread.c:1293)
==13835==    by 0x12549A17: xkl_config_get_keyboard (xklavier_config_xkb.c:281)
==13835==    by 0x1254A347: xkl_xkb_activate_config_rec (xklavier_config_xkb.c:471)
==13835==    by 0x12338AB3: gkbd_keyboard_config_activate (in /usr/lib/libgnomekbd.so.2.0.0)
==13835==    by 0x11F2D060: apply_xkb_settings (gsd-keyboard-xkb.c:187)
==13835==    by 0x11F2C8C0: gsd_keyboard_manager_start (gsd-keyboard-manager.c:402)
==13835==    by 0x11F2C1E6: impl_activate (gsd-keyboard-plugin.c:78)
==13835==    by 0x405471: gnome_settings_plugin_info_activate (gnome-settings-plugin-info.c:506)
==13835==    by 0x404094: maybe_activate_plugin (gnome-settings-manager.c:93)
==13835==    by 0x99F8C8C: g_slist_foreach (in /usr/lib/libglib-2.0.so.0.1704.0)
==13835==    by 0x404271: gnome_settings_manager_start (gnome-settings-manager.c:271)
==13835==    by 0x403E91: main (main.c:312)"