The DVD for the hacker documentary film Freedom Downtime has tons of fun Easter eggs, but your example reminds me of my favorite which is a hidden audio track. The track is totally silent for the entire film apart from when a man named Dave Buchwald (who produced the DVD) happens to be on screen for a moment, and you hear him say, "hey, that's me!" Later, during the credits, when Dave's name comes up he says, "that's my name!" And that's the whole track.