Eggnog Soap

I don't think I would call that a motherboard, just a generic printed circuit board. Companies (or one of their humorless departments) may be hesitant to stick random things on their circuit board where someone might see it, so while some are around, they aren't too common in my experience. Although there are collections around of images found on the actual silicon chips and integrated circuits, which normally would not be visible without destructively opening the plastic the chip is embedded in and then using a microscope to look at it.

I was doing a little reverse engineering of a circuit board from Jack Deville - taking each component off and tracing the connections to get an idea how the board worked, and under the main chip was a message 'Concede Defeat, Retain Pride'. I can't imagine too many people would see that, but it made my day.

Records (albums and 45s) sometimes have messages scratched into the plastic surrounding the center label.