Comment 35 for bug 8678

This is what could happen:

The user sets up the typing monitor. It crashes or the user kills it. Then the
user disables the typing monitor in the keyboard preferences.

This code is executed:
  if (typing_monitor_pid > 0)
    kill (typing_monitor_pid, SIGKILL);

With the child-exited signal removed, typing_monitor_pid is not reset to 0. So
it might kill another process that got the pid that the typing monitor had.

Of course, this won't happen often, so it's far less critical than the current bug.