Especially since the third one is a developer mistake that, as a sys admin and developer, I've had to point out to developers not to do -- but for security reasons, not because fork is oh-so-super expensive (even though it can be).

Also, there is no "system" system call. "system" is a library call that forks and execs a shell to evaluate and execute a string. Having a sys admin that doesn't know the difference may be the biggest sys admin mistake you could make. There are a lot of library wrappers for system calls, but these are documented in section 2 of the man pages as system calls.