7. The Tunnel
The tunnel before a Robot Master's lair actually served multiple purposes. Not only was it a save point for the level, but it also allowed the game to transition from the stage data to the boss' data without overloading and crashing. However, the Famicom (Japanese NES) had more than enough RAM capacity, so the tunnel was actually unnecessary in Japan. It was kept in later games as part of the tradition of Mega Man, even when the transition time was no longer needed (And for the people who couldn't defeat the Robot Masters in one go).