A particular sequence of object creations and property assignment segfaults in ZE2 with --enable-debug.
I verified this on latest CVS, with a vanilla "configure --enable-debug" build, running the CLI version.

This causes the segfault:

php -r 'class x { var $y; function z(){}} $a = new x; $a->y=1; $b = new x; $b->y=2; echo "all ok\n";'

Just swapping two statements in there makes it work fine:

php -r 'class x { var $y; } $a = new x; $a->y=1; $b = new x; $b->y=2; echo "all ok\n";'

This is what I can see in gdb:

Program received signal EXC_BAD_ACCESS, Could not access memory.
0x0017532c in _efree (ptr=0x5a5a5a5a, __zend_filename=0x1cf824 "/usr/local/book/php4/Zend/zend_object_handlers.c", __zend_lineno=88, __zend_orig_filename=0x0, __zend_orig_lineno=0) at /usr/local/book/php4/Zend/zend_alloc.c:219
219             CALCULATE_REAL_SIZE_AND_CACHE_INDEX(p->size);
(gdb) bt
#0  0x0017532c in _efree (ptr=0x5a5a5a5a, __zend_filename=0x1cf824 "/usr/local/book/php4/Zend/zend_object_handlers.c", __zend_lineno=88, __zend_orig_filename=0x0, __zend_orig_lineno=0) at /usr/local/book/php4/Zend/zend_alloc.c:219
#1  0x001a8ae0 in zend_std_write_property (object=0x5a36d8, member=0x5a222c, value=0x5a25a8) at /usr/local/book/php4/Zend/zend_object_handlers.c:88
#2  0x001b6384 in zend_assign_to_object (result=0x5a21f4, op1=0x5a220c, op2=0x5a2224, value=0x5a25a8, Ts=0xbffff1e8) at /usr/local/book/php4/Zend/zend_execute.c:304
#3  0x001af460 in execute (op_array=0x5a1978) at /usr/local/book/php4/Zend/zend_execute.c:1520
#4  0x00185eb8 in zend_eval_string (str=0xbffffc45 "class x { var $y; function z(){}} $a = new x; $a->y=1; $b = new x; $b->y=2; echo \"all ok\\n\";", retval_ptr=0x0, string_name=0x1d076c "Command line code") at /usr/local/book/php4/Zend/zend_execute_API.c:729
#5  0x001b9090 in main (argc=3, argv=0xbffffb78) at /usr/local/book/php4/sapi/cli/php_cli.c:725
#6  0x00002714 in _start ()
#7  0x00002544 in start ()