Semi Frequent Undertale Factsさんのイラストまとめ

* If you let Monster Kid fall in Waterfall, Undyne will save them. This will cause her to lose some HP which will carry over when you fight her.

Submitted by @SuperKirbylover

* Toriel reveals she wanted to be a teacher in Undertale, something her Deltarune counterpart achieves.

Sources: https://t.co/q8BOYkalST
I couldn't find the original post (probably doesn't exist anymore since I searched every account), but I at least found a high quality version of it
https://t.co/KptteUyjUc

* As seen by two very early in development Undertale drawings made by Temmie Chang, apparently Mettaton had claw hands instead of gloves.

* You can run away from the Dummy pretty much skipping the battle tutorial itself.

* Undertale never reveals why Frisk fell, however the kickstarter page does, saying Frisk was "Seeking refuge from the rain" which then leds in Frisk falling into the hole after trying to get a better look. (Second picture is tecnically Chara but you get the idea.)