This has probably been discussed before, but I still just wanna talk about it because I find it interesting.
So, Sans' genocide fight. Everyone's familiar with it. He's the one opponent that dodges your attacks. Until he doesn't. The one time a clean hit is landed on him is when it isn't the player attacking. That made me realize he's more likely reacting to the player's inputs, not the human. Every time he says "you" he's talking to you the player (e.g. "That's your doing, isn't it?" when he talks about timelines abruptly ending, and the "If we really are friends, you won't come back."). He's connected the human's actions and the resetting and the destruction of everything to the player. He knows the human is stuck going along with this the same way he and everyone else is. He quite obviously sends attacks around the menu when it's not his turn. He's willing to do the dance with the player until they likely get bored and move on, and, even after falling asleep, he reacts to the player's moving of the border box and pressing the fight button. But right after, when the human finishes the job on their own? He doesn't expect that. He doesn't see it coming because it was out of the player's control.
TL;DR Something pretty clear in retrospect and i'm not sure why I didn't realize until now is that Sans likely reacts to and interacts with the player, not the human, and the only reason we win is because he wasn't expecting the human to attack without player input. Also, I love this game.
