I wouldn't call the goofy stuff that I do in software Easter-eggs exactly, more like shenanigans. For example one time I put a do not press button on a screen when we were testing a machine at our facility. All it did was sound the horn on the machine as if it were going into automatic mode, but it didn't really do anything. One of our assembly guys called me a few hours later and complained that every time he tried to put the machine into automatic, it would do nothing after the horn was done sounding. I say: "You were pressing the do not press button weren't you?". I get a 5 second pause, then: "a$$hole", click.
My shenanigans get me in trouble once in a while. No humor.