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 back trace, we found:

Program received signal SIGSEGV, Segmentation fault.
0x000000000061c91b in ?? ()
#8 0x00007fd3d0993359 in ?? () from /xxxx/xxxx/opcache.so
#9 0x00007fd3d0994187 in ?? () from /xxxx/xxxx/opcache.so

This shows us the problem is in the opcode cache. The user upgraded the opcode cache to the newest version and tried a different version of PHP. When these did not fix the problem, the user disabled the opcode cache to allow the application to function normally.