In the episode "The Trouble with Tribbles," about two thirds to three fourths of the way through the episode there is a shot of the space station's bar. In this scene you see many tribbles lying around, but there is a special pile litting on the bar counter, this pile is special because you can see a man's face in the middle of the pile! You might have to look closely, because it is painted the same color of the tribbles, but it is there.