That bug has been opened on https://launchpad.net/ubuntu/+source/libwnck/+bug/78406

"....
Wasn't doing anything in particular to make this happen

http://librarian.launchpad.net/5626945/_usr_bin_gnome-panel.1001.crash
crash report
...
 Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
 Core was generated by `gnome-panel --sm-config-prefix /gnome-panel-haWrw2/ --sm-client-id 110a4b160100'.
 Program terminated with signal 8, Arithmetic exception.
 #0  0xb517eb11 in get_workspace_rect (pager=0x82b8d68, space=0, 
     rect=0xbfe0143c) at pager.c:586
 	in pager.c
 #0  0xb517eb11 in get_workspace_rect (pager=0x82b8d68, space=0, 
     rect=0xbfe0143c) at pager.c:586
 	hsize = -1
 	vsize = 44
 	n_spaces = <value optimized out>
 	widget = (GtkWidget *) 0x82b8d68
 	focus_width = 1
 	__PRETTY_FUNCTION__ = "get_workspace_rect"
 #1  0xb517f4d8 in wnck_pager_queue_draw_workspace (pager=0x82b8d68, i=0)
     at pager.c:1205
 	rect = {x = -1256717634, y = -1217231240, width = 137071976, 
   height = -1075833768}
 #2  0xb77069a9 in IA__g_cclosure_marshal_VOID__VOID (closure=0x8485770, 
     return_value=0x0, n_param_values=1, param_values=0xbfe0168c, 
     invocation_hint=0xbfe0159c, marshal_data=0xb5180290) at gmarshal.c:77
 	data1 = (gpointer) 0x830f390
 	data2 = (gpointer) 0x0
 	__PRETTY_FUNCTION__ = "IA__g_cclosure_marshal_VOID__VOID"
 #3  0xb76f961b in IA__g_closure_invoke (closure=0x8485770, return_value=0x0, 
     n_param_values=1, param_values=0xbfe0168c, invocation_hint=0xbfe0159c)
     at gclosure.c:490
 	marshal = (
     GClosureMarshal) 0x8061dc4 <g_cclosure_marshal_VOID__VOID@plt>
 	marshal_data = (gpointer) 0x0
 	__PRETTY_FUNCTION__ = "IA__g_closure_invoke"
 #4  0xb7709a13 in signal_emit_unlocked_R (node=0x810c908, detail=0, 
     instance=0x830f390, emission_return=0x0, instance_and_params=0xbfe0168c)
     at gsignal.c:2440
 	tmp = <value optimized out>
 	handler = (Handler *) 0x848a6c0
 	accumulator = (SignalAccumulator *) 0x0
 	emission = {next = 0x0, instance = 0x830f390, ihint = {
     signal_id = 205, detail = 0, run_type = G_SIGNAL_RUN_FIRST}, 
   state = EMISSION_RUN, chain_type = 4}
 	class_closure = (GClosure *) 0x810c8d8
 	handler_list = (Handler *) 0x83c3600
 	return_accu = (GValue *) 0x0
 	accu = {g_type = 0, data = {{v_int = 0, v_uint = 0, v_long = 0, 
       v_ulong = 0, v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, 
       v_pointer = 0x0}, {v_int = 0, v_uint = 0, v_long = 0, v_ulong = 0, 
       v_int64 = 0, v_uint64 = 0, v_float = 0, v_double = 0, v_pointer = 0x0}}}
 	signal_id = 205
 	max_sequential_handler_number = 7015
 	return_value_altered = 1
...
"