Undertale has two secret endings that can be seen by doing a Spoiler:Pacifist Run after a Genocide Run. If the player chooses to stay with Toriel, Chara's face appears on Frisk and a lower pitched version of Flowey's laugh plays. If the player chooses to leave, the picture of Frisk and friends is replaced with a picture of Chara in Frisk's place and big Xs appear on other characters. This may be implying that Chara killed them after the game. The only way to get the true pacifist ending back is to either modify or replace the game's save files.
Contributed by Cavery210
Video