David Gerrold didn't get to appear in any on-screen TOS material until TMP (he was an extra).
Wrong!
He added himself ("a security guard who looks a lot like a certain science fiction writer") to the cast of "More Tribbles, More Troubles" and Filmation complied. When Alan Dean Foster novelized the TAS episode, he was unimpressed by the cute stage direction in the script and decided to get one back on Gerrold - and called the unnamed redshirt "Ensign Hacker".
