Board Thread:Questions and Answers/@comment-31243556-20170206052624/@comment-27701762-20170206061923

Everything you described is the Neutral route leading into the True Pacifist route, and all of that can be redone infinitely. A True Reset undoes everything with the exception of the completion of a Genocide playthrough, which means that upon playing through the game again you will encounter Sans in the Last Corridor, and he will deliver whatever judgment corresponds to your current LV and number of kills.

Technically Sans does not appear on the True Pacifist route at all, as the True Pacifist route is defined as the specific events that can only occur on a True Pacifist playthrough, which means that essentially the True Pacifist route does not begin until the True Lab. Once you have gone through the True Lab and reenter New Home (or enter it for the "first time," if you started over and befriended all of the characters before entering New Home), Sans does not appear in the Last Corridor. But he still appears on the Neutral route and all that.

So you can do your "perfect playthrough" as many times as you like.