in undertale, there is an unused string of text that says "big boner down the lane"

If you talk to one of the frogs in the ruins you can change the color the spare text turns when you can spare from yellow to pink

Guess I can bring this little factoid back

The multi-hit weapon noises came from a snoopy cartoon

Papyrus's Shed and Napstablook's house have the same base layout In the artbook, Toby Fox mentioned that he was going to change it, but didn't get to it

scarlet forest has an exact quote of gaster's theme

Undertale

If you go to the top floor of Alphys's lab right before you enter the True Lab (after the Alphys date), you can find Mettaton undergoing repairs

an unused line of text says "bepis valley granola bars", an error line of text has papyrus saying "THAT BEPIS YOU'RE WEARING" and of course the wait a year before uploading the spritesheets text was changed to

If you reverse and slow the sound Mystery Man makes when he dissappears, its actually Gaster's theme.