Are you still adding printf/puts calls and restarting your app to figure what went wrong? Sometimes, the problem is hard to reproduce, or you only discover it in production. You've got a process that exhibits the bug, but you didn't run it under ruby-debug, so there's no choice but kill it and reproduce after adding some code to inspect your program, right?

Sure not. Jamis Buck blogged about how to use GDB to inspect a live Ruby process, and showed how to get a stack-trace using Ruby's C API and some GDB scripting:

(gdb) set $ary = (int)backtrace(-1)
(gdb) set $count = *($ary+8)
(gdb) set $index = 0
(gdb) while $index < $count
>  x/1s *((int)rb_ary_entry($ary, $index)+12)
>  set $index = $index + 1
>end

But it gets much easier than that. How about this:

(gdb) eval "caller"