• Gaster being trapped in the void, being forgotten by everyone and being Sans and Papyrus's father

Since Gaster is mysterious figure, it naturally gives a lot of room for many headcanons. However, headcanons about Gaster are so popular that they are considered as canon by many many fans. In thr canon, none of this stuff is ever confirmed or even hinted. Sure the game hints that Sans and Papyrus have a connection to Gaster but not specifically that Gaster is their father. It been also never stated that Gaster is trapped in the void and such concept isn't even mentioned in the game. All we know about Gaster's fate is that he "shattered across time and space" not that he was trapped in the void. And nothing confirms or even hints that all of the monsters forgot him just because no one mention him. Besides, his followers remember him and say that Asgore took a long time before hiring a new scientist as Gaster's brillance was "irreplaceable", suggesting that Asgore remembered Gaster even after his disappearance.

• Chara having red eyes and possesing Frisk in genocide run

While this one is pretty obviously wrong, many people claim that this all canon and often use depict Chara having red eyes and possesing Frisk in genocide run or forcing them to kill everyone while it's clearly not canon. In the game, Chara has BROWN eyes until the very end of the soulless pacifist run where the red eyes indicate that they have malicious intent. Yes, they have red eyes for ONE scene while they make two long dialogues at the end of the genocide run with BROWN eyes. And it's never confirmed that Chara possess Frisk in genocide run and it's even contradicted by the fact that Chara is HELPING Frisk to kill monsters by telling them how many are left to kill and they tell them to "keep attacking" when you check Sans, all of this prove that Frisk is in control in genocide run.

• Papyrus being an innocent cinnamon roll

Papyrus is clearly a nice person and even tells you that you can change in genocide run (and it often works) but that doesn't make him a completely innocent cinnamon roll who can't hurt a fly. Remember that in the game, he wants to be a part of the royal guard and he knows that it includes to fight humans. On the top of this, he fights a child with a hinch of their life at least three times and only spares them when they have only 1 hp left and so to capture them by force for selfish motives (to join the royal guard). This is far from this perfect innocent cinnamon roll everyone depict as.

• Sans remembering resets, having a flame in one eye and wearing Papyrus's scarf

Since Sans is aware of resets and can guess that Frisk reset before, many people concluded that Sans remembers previous resets, which is again, contradicted by the game itself. Sans never "remembers" resets, he just guess that Frisk is a time traveller because their facial expressions and behaviour indicate so. Also, if Sans was a time traveller, that wouldn't make sence for him to tell Frisk to "not tell other Sanses" that they were never friends because if he remembered the previous timelines, he would just remember it.

And Sans's eye GLOWS, it doens't has any blue flame when you fight him in genocide run and he only glows it when he use telekinesis magic and Sans never wears Papyrus's scarf when you kill the latter.

• Frisk being an innocent cinnamon roll

This is a a concept made to contrast Frisk with Chara, who's believed to be irredeemably evil by some fans and forcing Frisk to kill people. In the game, it's never confirmed that Frisk is such a pacifist kid. They can do many many mean things even in pacifist run where they can beat every monster who attacks them, tell Snowdrake that no one will ever love him the way he's etc...They can kill nearly all monster they meet and yet the mirror still says that "despite everything, it's still you" (Frisk). All of this prove that Frisk isn't necessarily a pacifist innocent kid as their actions and this idea don't match.

Remember that it doens't mean you should stop using these headcanons in your fanwork, just keep in mind that they don't reflect the canon.