I received the attached report via the Debian bug tracking system.
It is reproducible on the trunk as well as in EMACS_22_BASE, backtrace
in EMACS_22_BASE looking like this:

Program received signal SIGSEGV, Segmentation fault.
0x0806fb9b in decode_mode_spec_coding (coding_system=137604633, buf=0x0, eol_flag=0) at xdisp.c:17622
(gdb) backtrace
#0  0x0806fb9b in decode_mode_spec_coding (coding_system=137604633, buf=0x0, eol_flag=0) at xdisp.c:17622
#1  0x080772a3 in display_mode_element (it=0xbf86f554, depth=4, field_width=0, precision=-1, elt=137737323, props=137480393, risky=0) at xdisp.c:18043
#2  0x08077cde in display_mode_element (it=0xbf86f554, depth=3, field_width=0, precision=-1, elt=137925469, props=137480393, risky=0) at xdisp.c:17190
#3  0x08077cde in display_mode_element (it=0xbf86f554, depth=1, field_width=0, precision=0, elt=137923285, props=137480393, risky=0) at xdisp.c:17190
#4  0x0807b457 in Fformat_mode_line (format=137923269, face=137480393, window=137480393, buffer=137512260) at xdisp.c:17418
#5  0x08154fee in Feval (form=139449061) at eval.c:2326
#6  0x08155afc in Ffuncall (nargs=2, args=0xbf86f9d0) at eval.c:2985
#7  0x0817faac in Fbyte_code (bytestr=136611403, vector=136611420, maxdepth=<value optimized out>) at bytecode.c:679
#8  0x08155486 in funcall_lambda (fun=136611372, nargs=1, arg_vector=0xbf86fb24) at eval.c:3172
#9  0x08155890 in Ffuncall (nargs=2, args=0xbf86fb20) at eval.c:3042
#10 0x0817faac in Fbyte_code (bytestr=136602331, vector=136602348, maxdepth=<value optimized out>) at bytecode.c:679
#11 0x08155486 in funcall_lambda (fun=136602308, nargs=0, arg_vector=0xbf86fc64) at eval.c:3172
#12 0x08155890 in Ffuncall (nargs=1, args=0xbf86fc60) at eval.c:3042
#13 0x0817faac in Fbyte_code (bytestr=136599339, vector=136599356, maxdepth=<value optimized out>) at bytecode.c:679
#14 0x08155486 in funcall_lambda (fun=136599316, nargs=0, arg_vector=0xbf86fd30) at eval.c:3172
#15 0x08155680 in apply_lambda (fun=136599316, args=137480393, eval_flag=1) at eval.c:3096
#16 0x08154d44 in Feval (form=139177165) at eval.c:2376
#17 0x080f4e13 in top_level_2 () at keyboard.c:1339
#18 0x08154480 in internal_condition_case (bfun=0x80f4e00 <top_level_2>, handlers=137525897, hfun=0x80f8590 <cmd_error>) at eval.c:1483
#19 0x080f7980 in top_level_1 () at keyboard.c:1347
#20 0x0815455a in internal_catch (tag=137522081, func=0x80f7950 <top_level_1>, arg=137480393) at eval.c:1223
#21 0x080f83a9 in command_loop () at keyboard.c:1304
#22 0x080f876a in recursive_edit_1 () at keyboard.c:1007
#23 0x080f8856 in Frecursive_edit () at keyboard.c:1068
#24 0x080ee620 in main (argc=5, argv=0xbf8703e4) at emacs.c:1769

Lisp Backtrace:
"format-mode-line" (0x8388ac5)
"eval" (0x84fd2e5)
"command-line-1" (0x84b8375)
"command-line" (0x8572363)
"normal-top-level" (0x831c8c9)
(gdb)