The only easter egg I slipped into a program was a random connection string. When our product connected to the server, it printed a bunch of debug statements, most of which were redundant or pointless. I trimmed out all of those that I could, but ran into one string that was both redundant and pointless, but I couldn't remove it because of how the architecture worked.
So instead of leaving such a pointless debug message, I wrote a function that returned a random sentence. There were 3 columns of words to choose from: verb, adjective, noun, all filled with technical sounding words. So when our product connected and you had expanded the "details" view to see what it was doing, you'd see a line in there with random gibberish like "Enabling advanced algorithms", "Searching for obstructed scanners", and "Shifting exhausting transients".
The lead tester noticed it and threatened to make me take it out, but she ended up letting it slide for some reason. Must have been in a good mood that day.