Here it is the result of backtrace. The problem appears when using long names in input fields, but just if you post them as multipart/form-data

0x404ed740 in _efree (ptr=0x0, __zend_filename=0x4053f980 "/usr/src/php-5.0.0b4/ext/mbstring/mbstring.c", __zend_lineno=3391, __zend_orig_filename=0x0,
    __zend_orig_lineno=0) at /usr/src/php-5.0.0b4/Zend/zend_alloc.c:257
257             CALCULATE_REAL_SIZE_AND_CACHE_INDEX(p->size);


#0  0x404ed740 in _efree (ptr=0x0, __zend_filename=0x4053f980 "/usr/src/php-5.0.0b4/ext/mbstring/mbstring.c", __zend_lineno=3391, __zend_orig_filename=0x0,
    __zend_orig_lineno=0) at /usr/src/php-5.0.0b4/Zend/zend_alloc.c:257
#1  0x40383c47 in php_mb_gpc_encoding_converter (str=0x40c019fc, len=0x40c01b10, num=50, encoding_to=0x0, encoding_from=0x0)
    at /usr/src/php-5.0.0b4/ext/mbstring/mbstring.c:3391
#2  0x404c982a in php_mb_flush_gpc_variables (num_vars=50, val_list=0x40c019fc, len_list=0x40c01b10, array_ptr=0x40bf7d40) at /usr/src/php-5.0.0b4/main/rfc1867.c:62
#3  0x404cb109 in rfc1867_post_handler (content_type_dup=0x40bf6384 "multipart/form-data; boundary=", '-' <repeats 27 times>, "7d412f2f1160128", arg=0x40bf7d40)
    at /usr/src/php-5.0.0b4/main/rfc1867.c:841
#4  0x404c79cc in sapi_handle_post (arg=0x40bf7d40) at /usr/src/php-5.0.0b4/main/SAPI.c:114
#5  0x40384995 in mbstr_treat_data (arg=0, str=0x0, destArray=0x0) at /usr/src/php-5.0.0b4/ext/mbstring/mb_gpc.c:117
#6  0x404ce42e in php_hash_environment () at /usr/src/php-5.0.0b4/main/php_variables.c:582
#7  0x404c19a1 in php_request_startup () at /usr/src/php-5.0.0b4/main/main.c:1084
#8  0x40534dc8 in php_apache_request_ctor (r=0x8229900, ctx=0x822fb18) at /usr/src/php-5.0.0b4/sapi/apache2handler/sapi_apache2.c:442
#9  0x40535331 in php_handler (r=0x8229900) at /usr/src/php-5.0.0b4/sapi/apache2handler/sapi_apache2.c:521
#10 0x08099596 in ap_run_handler (r=0x8229900) at config.c:195
#11 0x08099aae in ap_invoke_handler (r=0x8229900) at config.c:401
#12 0x080748b3 in ap_process_request (r=0x8229900) at http_request.c:288
#13 0x08070b01 in ap_process_http_connection (c=0x82237f8) at http_core.c:293
#14 0x080a21ae in ap_run_process_connection (c=0x82237f8) at connection.c:85
#15 0x0809814c in child_main (child_num_arg=-36) at prefork.c:696
#16 0x080982f6 in make_child (s=0x812fd28, slot=0) at prefork.c:736
#17 0x0809834f in startup_children (number_to_start=5) at prefork.c:808
#18 0x08098a41 in ap_mpm_run (_pconf=0x80dd0a8, plog=0x811f1b0, s=0x812fd28) at prefork.c:1024
#19 0x0809d75a in main (argc=2, argv=0xbffffd74) at main.c:660

Please try using this CVS snapshot:

  http://snaps.php.net/php5-latest.tar.gz
 
For Windows:
 
  http://snaps.php.net/win32/php5-win32-latest.zip

ok! it seems to be resolved in the snapshot version
many thanks!