GoldenEye is a classic game for Nintendo 64. While most gamers must think that all it's secrets had been discovered, they would be wrong. Or at least they would have been wrong. In 2012, a fan discovered an Easter Egg that Rare had hidden in the game’s code.
The egg was a fully functional for the ZX Spectrum system, complete with ten games. Considering it took so long to find this hidden Easter egg, it makes sense that the explanation of just how to find it is rather complex.
It turns out the emulator was designed as a side project. The designers did it to see if such emulation was even possible on the N64. GoldenEye was being made at the time so they decided to hide the emulator in there. Rather than remove the egg from the game before it was shipped, it was instead disabled. A fan created a patch that re-enabled it.