You wanna know what I just realized?
After reading Fandom Wiki.

So have you ever wondered why Error hated Core!Frisk so much?

Well, after reading Error's wiki page, IIIIIIIIIIididntfindtheanswer...

But I did find this.

Geno.

Genocide from the Aftertale AU.

Is just one of the many redesigns of Error!Sans and was made into an AU soon after.

So this means.

That Geno is a mistake.

...

...

Reaper: GEEEEEEEEENOOOOOOOOOOOO-!
Geno: Dammit.