If you thought coding games was difficult, then how about doing that from within SNES classic Super Mario World? That's exactly what SethBling, p4plus2 and MrCheeze did to implement a version of Flappy Bird.
To save me the difficulty of explaining how it was done, Seth has released a video showing how each stage of the Super Mario World code injection was completed. It was a task which required pixel perfect precision and took around an hour to complete.
You can check out the full hour-long broadcast over on Twitch, plus Seth has kindly released his notes which could go some way to helping others achieve similar code injections.