Star Trek: Deep Space 9 Easter Egg - Author in a Shot

Watch the scene where Sisko and Dax turn a corner into a corridor with tribbles all over the floor. The grey haired security guard is David Gerrold, writer of the original "The Trouble with Tribbles".

And the tribble he is holding is the ONLY original Tribble used on the whole show, the rest were new ones made for the episode.

(Details taken from Gerrold's intro to the novelisation of the episode)