Description:
------------
A few things:

* It happens when running a specific "simpletest" integration test 
* It doesn't always happen, roughly 33-50% of the times
* Never happened with 5.3.2, I got a report from Bamboo as soon as I upgraded to 5.3.3RC3

Of course I can't get a simple reproduce script as the aforementioned test does tons of things, but of course I can provide more information, SSH access, or try anything I'm asked to.

Test script:
---------------
n/a

Expected result:
----------------
No failure

Actual result:
--------------
zend_mm_heap corrupted exit message, with the following backtrace

#0  0x000000000079f25b in zval_scan (pz=0x3b31970) at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend_gc.c:485
        p = (Bucket *) 0x3661108
#1  0x000000000079f6b9 in gc_collect_cycles () at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend_gc.c:535
        p = (zval_gc_info *) 0xee5ee0
        q = (zval_gc_info *) 0x0
        orig_free_list = (zval_gc_info *) 0x7fffffffc6e0
        orig_next_to_free = (zval_gc_info *) 0x211ef18
        count = 0
#2  0x000000000079fbd8 in gc_zval_possible_root (zv=0x33588b0) at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend_gc.c:166
        newRoot = (gc_root_buffer *) 0x3627830
#3  0x00000000007a4fde in zend_assign_to_object (result=0x211ef18, object_ptr=0xe567a0, property_name=0x211ef60, value_op=0x211efb0, Ts=0x113b228, opcode=136) at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend_execute.c:602
        object = (zval *) 0x3632b70
        free_value = {var = 0x113b701}
        value = (zval *) 0x33588b0
        retval = (zval **) 0x113b6e0
#4  0x00000000007e2796 in ZEND_ASSIGN_OBJ_SPEC_UNUSED_CONST_HANDLER (execute_data=0x113b190) at zend_vm_execute.h:17645
        opline = (zend_op *) 0x0
#5  0x00000000007a65f9 in execute (op_array=0x2119968) at zend_vm_execute.h:107
        ret = 0
        execute_data = (zend_execute_data *) 0x113b190
        nested = 1 '\001'
        original_in_execution = 1 '\001'
#6  0x0000000000777d94 in zend_call_function (fci=0x7fffffffc970, fci_cache=0x0) at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend_execute_API.c:963
        call_via_handler = 34934168
        i = 18062328
        original_return_value = (zval **) 0x1139bf8
        calling_symbol_table = (HashTable *) 0x0
        original_op_array = (zend_op_array *) 0x2150d98
        original_opline_ptr = (zend_op **) 0x1139f28
        current_scope = (zend_class_entry *) 0x2118528
        current_called_scope = (zend_class_entry *) 0x2104658
        calling_scope = (zend_class_entry *) 0x2104658
        called_scope = (zend_class_entry *) 0x2104658
        current_this = (zval *) 0x30c9840
        execute_data = {opline = 0x0, function_state = {function = 0x2109b78, arguments = 0x113a068}, fbc = 0x0, called_scope = 0x0, op_array = 0x0, object = 0x3632b70, Ts = 0x1139fe0, CVs = 0x1139fc0, symbol_table = 0x0,
  prev_execute_data = 0x1139f28, old_error_reporting = 0x0, nested = 1 '\001', original_return_value = 0x2104658, current_scope = 0x30c9840, current_called_scope = 0x0, current_this = 0x0, current_object = 0x0, call_opline = 0x1139fc8}
#7  0x0000000000728986 in xml_call_handler (parser=0x2f77938, handler=0x3356688, function_ptr=0x3627830, argc=3, argv=0x7fffffffca50) at /array1/compile/php-5.3.3RC3-fcgi/ext/xml/xml.c:530
        args = (zval ***) 0x2f7e210
        retval = (zval *) 0x0
        result = -13744
        fci = {size = 72, function_table = 0xe58180, function_name = 0x3356688, symbol_table = 0x0, retval_ptr_ptr = 0x7fffffffc968, param_count = 3, params = 0x2f7e210, object_ptr = 0x3632b70, no_separation = 0 '\0'}
        i = 3
#8  0x000000000072926a in _xml_startElementHandler (userData=0x2f77938, name=0x11fa8c0 "plugin", attributes=0x0) at /array1/compile/php-5.3.3RC3-fcgi/ext/xml/xml.c:822
        attrs = (const char **) 0x0
        att = 0x0
        val = 0x11fa8c0 "plugin"
        val_len = 0
        retval = (zval *) 0x821ae6ce
        args = {0x37ba0f0, 0x3359b18, 0x37ba450}
#9  0x000000000072b56e in _start_element_handler (user=0x2d40860, name=0x11fa8c0 "plugin", attributes=0x0) at /array1/compile/php-5.3.3RC3-fcgi/ext/xml/compat.c:84
        qualified_name = (xmlChar *) 0x11fa8c0 "plugin"
#10 0x00000000820fa26a in xmlParseStartTag () from /usr/local/lib/libxml2.so.5
No symbol table info available.
#11 0x00000000820ff102 in xmlParseTryOrFinish () from /usr/local/lib/libxml2.so.5
No symbol table info available.
#12 0x00000000821004ab in xmlParseChunk () from /usr/local/lib/libxml2.so.5
No symbol table info available.
#13 0x000000000072c00d in php_XML_Parse (parser=0x2d40860, data=0x3540020 "", data_len=56784944, is_final=0) at /array1/compile/php-5.3.3RC3-fcgi/ext/xml/compat.c:605
        error = 0
#14 0x000000000072a963 in zif_xml_parse (ht=62069104, return_value=0x374c980, return_value_ptr=0x3627830, this_ptr=0x0, return_value_used=0) at /array1/compile/php-5.3.3RC3-fcgi/ext/xml/xml.c:1464
        parser = (xml_parser *) 0x2f77938
        pind = (zval *) 0x374ccf0
        data = 0x3356e18 "<?xml version=\"1.0\" encoding=\"ISO-8859-1\" ?>\n<?xml-stylesheet type=\"text/xsl\" href=\"\"?>\n\n<plugin>\n    <name>apRetargetingDriverExternalUI</name>\n    <creationDate>2010-06-10</creationDate>\n    <author"...
        data_len = 1075
        ret = 0
        isFinal = 1
#15 0x00000000007a7100 in zend_do_fcall_common_helper_SPEC (execute_data=0x1139f28) at zend_vm_execute.h:316
        i = 3
        p = (zval **) 0x113a048
        arg_count = 0
        opline = (zend_op *) 0x213f2b8
        should_change_scope = 0 '\0'
#16 0x00000000007a65f9 in execute (op_array=0x2150d98) at zend_vm_execute.h:107
        ret = 0
        execute_data = (zend_execute_data *) 0x1139f28
        nested = 1 '\001'
        original_in_execution = 0 '\0'
#17 0x0000000000785675 in zend_execute_scripts (type=8, retval=0x0, file_count=3) at /array1/compile/php-5.3.3RC3-fcgi/Zend/zend.c:1194
        files = {{gp_offset = 40, fp_offset = 48, overflow_arg_area = 0x7fffffffcf30, reg_save_area = 0x7fffffffce40}}
        i = 1
        file_handle = (zend_file_handle *) 0x7fffffffe850
        orig_op_array = (zend_op_array *) 0x0
        orig_retval_ptr_ptr = (zval **) 0x0
#18 0x0000000000735158 in php_execute_script (primary_file=0x7fffffffe850) at /array1/compile/php-5.3.3RC3-fcgi/main/main.c:2260
        realfile = "/usr/local/bamboo/test-home/xml-data/build-dir/RET-TRUNK/tests/run.php\000\000>@Ü\200\000\000\000\000\000\027Þ\200\000\000\000\0000áÿÿÿ\177\000\000\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000\t*¹\n\000\000\000\000é=Ü\200", '\0' <repeats 13 times>, "rÞ\200\000\000\000\000(áÿÿÿ\177\000\000\000\000\000\000\000\000\000\000páÿÿÿ\177\000\000ç\016", '\0' <repeats 14 times>, "\001\000\000\000\000\000\000\000\t*¹\n\000\000\000\000\001<Ü\200\000\000\000"...
        prepend_file_p = (zend_file_handle *) 0x0
        append_file_p = (zend_file_handle *) 0x0
        prepend_file = {type = ZEND_HANDLE_FILENAME, filename = 0x0, opened_path = 0x0, handle = {fd = 0, fp = 0x0, stream = {handle = 0x0, isatty = 0, mmap = {len = 0, pos = 0, map = 0x0, buf = 0x0, old_handle = 0x0, old_closer = 0},
      reader = 0, fsizer = 0, closer = 0}}, free_filename = 0 '\0'}
        append_file = {type = ZEND_HANDLE_FILENAME, filename = 0x0, opened_path = 0x0, handle = {fd = 0, fp = 0x0, stream = {handle = 0x0, isatty = 0, mmap = {len = 0, pos = 0, map = 0x0, buf = 0x0, old_handle = 0x0, old_closer = 0},
      reader = 0, fsizer = 0, closer = 0}}, free_filename = 0 '\0'}
        old_cwd = 0x7fffffffcf40 ""
        retval = 0
#19 0x00000000008099fb in main (argc=9, argv=0x7fffffffe948) at /array1/compile/php-5.3.3RC3-fcgi/sapi/cli/php_cli.c:1192
        len = 140737488348832
        argn = (zval *) 0x80de6600
        input = 0x0
        index = 9
        argi = (zval *) 0x80ee0030
        exit_status = 0
        c = 0
        file_handle = {type = ZEND_HANDLE_MAPPED, filename = 0x7fffffffeb75 "run.php", opened_path = 0x0, handle = {fd = 15152376, fp = 0xe734f8, stream = {handle = 0xe734f8, isatty = 0, mmap = {len = 5351, pos = 0, map = 0x80df4000,
        buf = 0x80df4000 <Address 0x80df4000 out of bounds>, old_handle = 0x8270d840, old_closer = 0x797cd0 <zend_stream_stdio_closer>}, reader = 0x797cb0 <zend_stream_stdio_reader>, fsizer = 0x797cf0 <zend_stream_stdio_fsizer>,
      closer = 0x797d50 <zend_stream_mmap_closer>}}, free_filename = 0 '\0'}
        behavior = 1
        reflection_what = 0x0
        orig_optind = 1
        orig_optarg = 0x0
        arg_free = 0x7fffffffeb75 "run.php"
        arg_excp = (char **) 0x3540020
        script_file = 0x7fffffffeb75 "run.php"
        interactive = 0
        module_started = 1
        request_started = 1
        lineno = 1
        exec_direct = 0x0
        exec_run = 0x0
        exec_begin = 0x0
        exec_end = 0x0
        param_error = 0x0
        hide_argv = 0
        ini_entries_len = -6496