Software Easter Eggs - What would you put?

[Amaroq_Starwind]

What kind of easter egg (hidden messages in a program or game; often for the purposes of humor, showing appreciation, or  dedicating to someone's memory) would you include in software that you develop?

When I was still trying to learn Python, I wrote a utility for tabletop BattleTech which would throw in-universe quips at you if you gave invalid input or out-of-bounds design parameters. For instance, if you tried putting an engine in your mech that was too heavy, it would tell you that the mech had collapsed under its own weight. One more easter egg I had included was that ot would say Out Of Cheese Error -- Redo From Start if you calculated a negative engine rating (a Terry Pratchett reference).

I was inspired to create this thread by previous discussion about the pirate easter egg in MalwareBytes.