Maybe this isn't quite an Easter Egg per se, but did you ever see something in a game and then later when you went back to do it again it was gone? At one time, I could have sworn that R&B had a sound test where you could select any sound to play back (NOT the "Speaker Test" where you just have the six preset sounds/music). I could never find it again and began to think I was just imagining it. However, I was just able to reproduce it and now I know how. You have to set Adjustment 45: Tournament Style to Home. Then, when you go into Diagnostics, there will be a new entry after "-Coil Test-" named "-Sound Test-".
Use the left or right flipper buttons to scroll through all 256 sound commands (00 to FF hex) and press Start to play the sound.
Not all commands are sounds. For example, 00 stops all playback (useful when you started a looping sound you want to stop). Also 20 (maximum) to 2F (minimum) are the volume for the background music. Finally, hold both flippers at the same time to increase the left digit (useful for getting to a far away sound command faster).
This also works in TFTC (v3.03). I tried it in my LAH, but it didn't work. If you are able to duplicate (or not) in another DE game, then please post it here.