User B complained of 503 errors while using a collaboration application. We used strace to find that PHP was crashing:

18:06:05.816339 stat("/xxxxx/xxxx/x/xxxx.class.php", {st_mode=S_IFREG|0666, st_size=54285, ...}) = 0 <0.000024>
18:06:05.817292 brk(0x16e6000) = 0x16e6000 <0.000026>
18:06:05.817790 --- SIGSEGV (Segmentation fault) @ 0 (0) ---

Using GNU Debugger to examine the stack backtrace, we found:

Program received signal SIGSEGV, Segmentation fault.
0x000000000061c91b in ?? ()
(gdb) bt
#0 0x000000000061c91b in ?? ()
#1 0x0000000000641ac3 in zend_stack_push ()
#2 0x000000000060bbf9 in ?? ()
#3 0x0000000000611515 in lex_scan ()
#4 0x000000000061fb60 in ?? ()
#5 0x0000000000608223 in ?? ()
#6 0x0000000000614965 in compile_file ()
#7 0x00007fd3c99eda21 in ?? () from /opt/alt/php55/usr/lib64/php/modules/phar.so
#8 0x00007fd3d0993359 in ?? () from /opt/alt/php55/usr/lib64/php/modules/opcache.so
#9 0x00007fd3d0994187 in ?? () from /opt/alt/php55/usr/lib64/php/modules/opcache.so
#10 0x00000000006b54da in ?? ()
#11 0x00000000006b68e8 in execute_ex ()
#12 0x000000000064239c in zend_execute_scripts ()
#13 0x00000000005e2fb0 in php_execute_script ()
#14 0x00000000006f29ff in ?? ()
#15 0x00000000006f2c5c in ?? ()
#16 0x00000000006f2f85 in ?? ()
#17 0x00007fd3d37cdd1d in __libc_start_main () from /lib64/libc.so.6
#18 0x0000000000424bc9 in _start ()

This shows us the problem is in the opcode cache. We do not know whether the user upgraded the opcode cache, tried a different version of PHP, or disabled the opcode cache, but the issue seems to have been resolved.