GoldenEye, for the Nintendo 64, is one of the all-time classics of gaming. Recently, a fan uncovered an Easter egg Rare had buried in GoldenEye's code: a fully functional emlulator for the ZX Spectrum system, complete with ten games.
Rare, the developer behind GoldenEye, developed a number of games in the early 1980s for the ZX Spectrum, a system similar to the Commodore 64 and primarily sold in the UK. (Rare is a UK-based company.)
The original post discussing how the emulator was found and how it can be accessed is fairly complex, outlining the process in heavily technical language. The gist of it all, however, is this: the emulator was designed as a side project, to see if such system emulation were possible on the N64. GoldenEye was the project in development at the time and so it was the game the emulator was tried with. Rather than being removed before the final game shipped, the code was instead only disabled, and a recent fan-developed patch has re-enabled it.
The easiest way to apply the patch, according to the post's author, is in an N64 emulator. That makes this Easter egg something like the Inception of game systems. It's emulators all the way down.