Undertale Remembers

Believe it or not, Undertale remembers what you did. In most games, if you mess up, you can reload like nothing happened, but in Undertale, you cannot do that. For instance if you name the fallen human and reset, you cannot choose another name, if Flowey encounters you maybe 3 or 4 times at the start
he will say "don't you have anything better to do"

If you kill Toriel and reload, she will say you look like you saw a ghost, you tell her you saw her die and she will say its impossible. And Flowey will mention that you killed Toriel, felt bad and went back. If you do a genocide run and reset, before you reach Snowdin, Sans will say "you're about to fight my brother soon, here is some advice, don't."

You know how Sans does one of his whoopee cushion tricks, if you reload he will ask you if you heard it already

When Mettaton sings to you, if he knows you saw his performance already, there will be an option to skip it. If you lose during the Mettaton EX fight, you will have to option to skip right back to the fight

At the fight with Asgore, if you die, you can tell him he killed you

After fighting Omega Flowey, if you go back to fight Asgore, Flowey will say the souls will rebel and he won't transform, when Flowey pops up he will ask you why you keep coming back.

At the Sans fight, each time you die, he has something to say for each death
If you get dunked on by Sans, and make it to the break, he will mention that you're pissed.

If you do a pacifist run after a genocide, you will get a creepy ending, as Chara has your soul. The game is like sike you thought you will get a happy ending after what you did? No you will pay for what you did If you go with Toriel, Chara will posses Frisk and you hear Flowey's laugh slowed down. If you say you have places to go, at the photo at the end, everyone will be crossed out and Chara will be in the picture.
well that's all I got
this video isn't mine but I will share it as it may cover things that I missed

the game is very clever, you cannot out smart it.