During the pacifist run, when you encounter Sans before the Asgore battle, you can have a bit of fun.

If you speak to him then quit the game without saving, then reload your save file and talk to him again a few times, he'll eventually give you a key.

Have fun backtracking all the way to Snowdin to unlock his room!

Then you walk.

And walk.

And walk.

Eventually you can take a key out of his drawer and use it to unlock a secret door behind the house (that I don't think even Pap knows about).

It leads to a scientist's workroom that has a blueprint and a rectangular object covered by a tarp in it. (I personally think it's a TARDIS. Hue hue hue)

The blueprint has strange markings on it, which some presume to be Wing Dings, since we never see the blueprint up close.