Board Thread:General Discussion/@comment-5413572-20170219072514/@comment-5413572-20170219162835

Except Sans (and for that matter, Asgore) clearly know about and reference events that have happened in other runs, such as how many times they've killed you. Sans in particular explicitly references the existence of other timelines and how Frisk's abilities and subsequent travel are wreaking havoc on them.

To be fair, Flowey also does state that resetting involves "ripping everyone from this timeline" and taking them back as well, but this in and of itself is questionable (and indeed may only apply with a True Reset). For example, if someone has been killed in a run, there's not really anything there to bring back, indicating that the reset involved going back to a point before they'd died rather than reconstituting them.

And either way, Gaster attempting an experiment with these mechanics could still provide the means to shatter and scatter him.