I have a smiley face that appears on an HMI for room temp control when the room temp is within 0.1 degree F of the setpoint. After I tweaked the controls and got the system to work really well, that smiley face appears all the time (except when it's over 100 outside).
I have several easter eggs in the video game I made for the G3, but in real life, very few, and all harmless ones.
At my previous employer, I had to make changes to some electrical drawings in cad, and the boss would not let me make all the wiring changes I wanted in a terrible relay logic circuit that we all hated. I make the little dots that connected all the electrical points together in the schematic into some spiral text that was basically a paragraph of highly critical venting (I ripped the designer a new one). When you print the drawing you can't see it, but if you open the autocad file and zoom in...