While writing a Z80 emulator, you might ask yourself: should it be 100% accurate ? Maybe not, if you just want to run old software. But what if you're not and this old software accidentally depends on undocumented behavior ? In this talk we'll explore how, 30 years after its design, a few people on a Russian retro-computing forum reverse engineered an undocumented Z80 register: MEMPTR. And how 6 years later, another even less known secret register was discovered.