"You're showing off how awesome you are at a game. You might be on a harder difficulty than you usually play. You might be playing the whole game without getting hit. You might even be playing the game blindfolded. (Okay, probably not.) Regardless, you get to the Final Boss, unleash complete destruction, and defeat it without much effort. You put the controller down, ready to watch the ending...
...and then frantically pick it up again when a new boss comes out of nowhere and starts kicking your ass. Huh? Where did this guy come from?! You don't know why this boss never showed up before. Maybe you had to be at the hardest difficulty level. Maybe you're doing much better than you were before. Maybe you completed a sidequest that didn't necessarily let you know this was going to happen as a result. Your journey has ended. Regardless, you have now run into the True End Boss."