OK. a Q&A with Sebastien helped me fill a bit of my knowledge gap. Alt-F2 (Run Application) can either run an application _or_ open a location. Since '#' is not an application, we end up with a call to nautilus: "nautilus --no-desktop file:///#".

And... this is enough to trigger the bug...

I will open a bug upstream for that, or link to an existing one.

gdb run :

hggdh@xango:~$ gdb --args nautilus --no-desktop file:///#
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This GDB was configured as "x86_64-linux-gnu"...
Using host libthread_db library "/lib/libthread_db.so.1".
(gdb) run
Starting program: /usr/bin/nautilus --no-desktop file:///\#
[Thread debugging using libthread_db enabled]
[New Thread 47899527502704 (LWP 19678)]
Initializing gnome-mount extension
seahorse nautilus module initialized
[New Thread 1082132800 (LWP 19699)]
[New Thread 1090525504 (LWP 19700)]
[New Thread 1098918208 (LWP 19701)]
[New Thread 1107310912 (LWP 19702)]
[New Thread 1115703616 (LWP 19703)]
[New Thread 1124096320 (LWP 19704)]
[New Thread 1132489024 (LWP 19705)]
[New Thread 1140881728 (LWP 19709)]
[New Thread 1149274432 (LWP 19710)]
[New Thread 1157667136 (LWP 19711)]
sys:1: Warning: g_filename_display_basename: assertion `filename != NULL' failed

** (nautilus:19678): CRITICAL **: nautilus_information_panel_set_uri: assertion `initial_title != NULL' failed
[Thread 1140881728 (LWP 19709) exited]
[Thread 1082132800 (LWP 19699) exited]
[Thread 1090525504 (LWP 19700) exited]
[Thread 1124096320 (LWP 19704) exited]
[Thread 1132489024 (LWP 19705) exited]
[Thread 1107310912 (LWP 19702) exited]
[Thread 1115703616 (LWP 19703) exited]
[Thread 1098918208 (LWP 19701) exited]
[Thread 1157667136 (LWP 19711) exited]
[Thread 1149274432 (LWP 19710) exited]
[New Thread 1149274432 (LWP 19715)]
[New Thread 1107310912 (LWP 19716)]
[New Thread 1124096320 (LWP 19717)]
[New Thread 1157667136 (LWP 19718)]

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 47899527502704 (LWP 19678)]
compare_by_display_name (file_1=0x1219de0, file_2=0x1223820) at nautilus-file.c:1829
1829    nautilus-file.c: No such file or directory.
        in nautilus-file.c
(gdb) backtrace
#0  compare_by_display_name (file_1=0x1219de0, file_2=0x1223820) at nautilus-file.c:1829
#1  0x00000000004c8dbe in nautilus_file_compare_for_sort (file_1=0x1219de0, file_2=0x1223820, 
    sort_type=NAUTILUS_FILE_SORT_BY_DISPLAY_NAME, directories_first=<value optimized out>, reversed=0) at nautilus-file.c:2182
#2  0x00002b907369c3d3 in g_list_sort_real (list=<value optimized out>, compare_func=0x479640 <compare_files_cover>, 
    user_data=0xe1c240) at glist.c:583
#3  0x00002b907369c39d in g_list_sort_real (list=0x1224800, compare_func=0x479640 <compare_files_cover>, user_data=0xe1c240)
    at glist.c:629
#4  0x00002b907369c39d in g_list_sort_real (list=0x1224920, compare_func=0x479640 <compare_files_cover>, user_data=0xe1c240)
    at glist.c:629
#5  0x00002b907369c39d in g_list_sort_real (list=0x1210640, compare_func=0x479640 <compare_files_cover>, user_data=0xe1c240)
    at glist.c:629
#6  0x0000000000475996 in sort_files (view=0x31, list=0xe32618) at fm-directory-view.c:2689
#7  0x000000000047990e in process_new_files (view=0xe1c240) at fm-directory-view.c:2767
#8  0x00000000004799ff in done_loading_callback (directory=<value optimized out>, callback_data=<value optimized out>)
    at fm-directory-view.c:3175
#9  0x00002b90734387da in IA__g_closure_invoke (closure=0x11f6e40, return_value=0x0, n_param_values=1, 
    param_values=0x7fff3b7122f0, invocation_hint=0x7fff3b7121c0) at gclosure.c:490
#10 0x00002b9073448408 in signal_emit_unlocked_R (node=0xb26660, detail=0, instance=0xb383b0, emission_return=0x0, 
    instance_and_params=0x7fff3b7122f0) at gsignal.c:2440
#11 0x00002b9073449843 in IA__g_signal_emit_valist (instance=0xb383b0, signal_id=<value optimized out>, detail=0, 
    var_args=0x7fff3b712570) at gsignal.c:2199
#12 0x00002b9073449a13 in IA__g_signal_emit (instance=0x2b9074837960, signal_id=19025024, detail=49) at gsignal.c:2243
#13 0x00000000004b14e8 in dequeue_pending_idle_callback (callback_data=0x0) at nautilus-directory-async.c:914
#14 0x00000000004b166a in directory_load_callback (handle=0x1a, result=GNOME_VFS_ERROR_EOF, list=0x12140a0, 
    entries_read=<value optimized out>, callback_data=0x0) at nautilus-directory-async.c:1041
#15 0x00002b907290654e in dispatch_job_callback (data=<value optimized out>) at gnome-vfs-job.c:237
#16 0x00002b907369ea14 in IA__g_main_context_dispatch (context=0x792ae0) at gmain.c:2045
#17 0x00002b90736a185d in g_main_context_iterate (context=0x792ae0, block=1, dispatch=1, self=<value optimized out>)
    at gmain.c:2677
#18 0x00002b90736a1b6a in IA__g_main_loop_run (loop=0x7c5180) at gmain.c:2881
#19 0x00002b9071466023 in gtk_main () from /usr/lib/libgtk-x11-2.0.so.0
#20 0x00000000004537a3 in main (argc=<value optimized out>, argv=0x7fff3b712db8) at nautilus-main.c:548
(gdb)