Extra hidden code that does something fun. Seems like a good idea, and generally speaking I think they're a simple, mostly harmless bit of entertainment. I've enjoyed them. I've built them. Falcon 4's shared memory output started out as a development tool then became an easter egg, and eventually a feature.
They're an old tradition. The first one I ran into was the "Identify 9" easter egg in the old Wizardry series. Try to identify the 9th item in an 8 item list and you got 100M exp points. Then there's the Konami code, which works in all sorts of strange places. And you can still find them in recent tools. The `opus` tool had a couple, `starwars` and `wizard`. You can find them in hardware too. From chip designers baking their initials into circuit boards to sneaking a Commodore64 program onto a rock album.
But more importantly, while they might be surprises to users, they shouldn't be surprises to the development team. Especially when you're working on mission critical or safety of flight sorts of things. And that's because they need to be tested. And not the standard developer testing of the happy path, but fully tested, including fuzz testing on any inputs and ensuring that they handle them gracefully.
For example.early versions of Microsoft Excel had a hidden flight simulator. It needed to be tested. Not to ensure that the flight model was high quality or that it obeyed physics, but that it wouldn't corrupt data. That it wouldn't suddenly crash and cause data loss. It's bad enough that people don't know the limits and loose data because they add too many rows to a spreadsheet. Image what would have happened if the data loss was caused by a bug in secret code. That wouldn't have gone over very well.
So don't be afraid to have some fun and to spark joy in your customers, but think it through, and don't do things in secret. That never ends well.