Board Thread:General Discussion/@comment-26006155-20181123023529/@comment-26006155-20181128181428

Also, I don't think Gaster and Sans "Fell", per say.

Remember the mysterious machine behing San's house?

Sans confirms in his fight dialog, as well as the sequence of restarting The Last Corridor you have to do in order to get The Key to San's Room, that he's a time traveler.

And what do we find, when we finally get San's key? And check Our Mysterious Key Chain that for some reason we are completely unable to look at?

We find the key was on our keychain all along.

We are Sans. Just an earlier version of him.