In parsing boolean options in smb.conf, the following code path is executed.

lib/util/util.c:
 790 _PUBLIC_ bool set_boolean(const char *boolean_string, bool *boolean)
 791 {          
 792         if (strwicmp(boolean_string, "yes") == 0 ||
 793             strwicmp(boolean_string, "true") == 0 ||
 794             strwicmp(boolean_string, "on") == 0 ||
 795             strwicmp(boolean_string, "1") == 0) {
 796                 *boolean = true;
 797                 return true;

lib/util_str.c:
 229 int strwicmp(const char *psz1, const char *psz2)
 230 {
...
 246                 if (toupper_ascii(*psz1) != toupper_ascii(*psz2) ||
 247                                 *psz1 == '\0' || *psz2 == '\0')
 248                         break;
...
260 int toupper_ascii(int c)
261 {
262         smb_ucs2_t uc = toupper_m(UCS2_CHAR(c));


lib/util/charset/charset.h
 42 #ifdef WORDS_BIGENDIAN
 43 #define UCS2_SHIFT 8
 44 #else
 45 #define UCS2_SHIFT 0
 46 #endif
 47 
 48 /* turn a 7 bit character into a ucs2 character */
 49 #define UCS2_CHAR(c) ((c) << UCS2_SHIFT)


lib/util/charset/codepoints.c
 90 _PUBLIC_ codepoint_t toupper_m(codepoint_t val)
 91 {
 92         if (val < 128) {
 93                 return toupper(val);
 94         }


On big endian systems, the ASCII val passed into toupper_m is pre-shifted meaning toupper() does not get called:
Breakpoint 1, set_boolean (boolean_string=0x10fb7d37 "Yes", boolean=0xfffffffe5f0) at ../lib/util/util.c:792
792             if (strwicmp(boolean_string, "yes") == 0 ||
(gdb) b toupper_m
Breakpoint 2 at 0x10681a80: file ../lib/util/charset/codepoints.c, line 92.
(gdb) c
Continuing.

Breakpoint 2, toupper_m (val=22784) at ../lib/util/charset/codepoints.c:92
92              if (val < 128) {
(gdb) p /x val
$1 = 0x5900
(gdb) l
87      /**
88       Convert a codepoint_t to upper case.
89      **/
90      _PUBLIC_ codepoint_t toupper_m(codepoint_t val)
91      {
92              if (val < 128) {
93                      return toupper(val);
94              }
95              if (upcase_table == NULL) {
96                      load_case_tables_library();
(gdb) bt
#0  toupper_m (val=22784) at ../lib/util/charset/codepoints.c:92
#1  0x000000001068169c in toupper_ascii (c=89) at lib/util_unistr.c:616
#2  0x0000000010677c38 in strwicmp (psz1=0x10fb7d37 "Yes", psz2=0x10da6618 "yes") at lib/util_str.c:246
#3  0x0000000010638bf8 in set_boolean (boolean_string=0x10fb7d37 "Yes", boolean=0xfffffffe5f0) at ../lib/util/util.c:792
#4  0x00000000100bcca0 in lp_bool (s=0x10fb7d37 "Yes") at param/loadparm.c:6168
#5  0x00000000100c3218 in lp_do_parameter (snum=-2, pszParmName=0x10fb7d20 "usershare allow guests", pszParmValue=0x10fb7d37 "Yes")
    at param/loadparm.c:7991
#6  0x00000000100c38b0 in do_parameter (pszParmName=0x10fb7d20 "usershare allow guests", pszParmValue=0x10fb7d37 "Yes", userdata=0x0)
    at param/loadparm.c:8100
#7  0x000000001063acc4 in Parameter (InFile=0x10fb7740, pfunc=@0x10eb8948: 0x100c37c8 <do_parameter>, c=0, userdata=0x0)
    at ../lib/util/params.c:426
#8  0x000000001063ae24 in Parse (InFile=0x10fb7740, sfunc=@0x10eb89c0: 0x100c3fcc <do_section>, 
    pfunc=@0x10eb8948: 0x100c37c8 <do_parameter>, userdata=0x0) at ../lib/util/params.c:482
#9  0x000000001063b234 in pm_process (FileName=0x10fb7230 "/etc/samba/smb.conf", sfunc=@0x10eb89c0: 0x100c3fcc <do_section>, 
    pfunc=@0x10eb8948: 0x100c37c8 <do_parameter>, userdata=0x0) at ../lib/util/params.c:566
#10 0x00000000100c8a0c in lp_load_ex (pszFname=0x10fb58f0 "/etc/samba/smb.conf", global_only=true, save_defaults=false, add_ipc=false, 
    initialize_globals=true, allow_include_registry=false, allow_registry_shares=false) at param/loadparm.c:9564
#11 0x00000000100c90a4 in lp_load_initial_only (pszFname=0x10fb58f0 "/etc/samba/smb.conf") at param/loadparm.c:9664
#12 0x0000000010c2014c in main (argc=7, argv=0xffffffff328) at smbd/server.c:1050


set_boolean() failures result in misinterpretation of smb.conf:
[2011/07/19 12:59:56.775424,  0] param/loadparm.c:6169(lp_bool)
  lp_bool(Yes): value is not boolean!
[2011/07/19 12:59:56.775526,  2] param/loadparm.c:8287(do_section)
  Processing section "[homes]"
[2011/07/19 12:59:56.775759,  0] param/loadparm.c:6169(lp_bool)
  lp_bool(No): value is not boolean!
[2011/07/19 12:59:56.775835,  0] param/loadparm.c:6169(lp_bool)
  lp_bool(No): value is not boolean!
[2011/07/19 12:59:56.775909,  0] param/loadparm.c:6169(lp_bool)
  lp_bool(Yes): value is not boolean!