Board Thread:Questions and Answers/@comment-30128098-20161002200635/@comment-26347196-20161002234011

Yea, I'm not sure about this either. For once, you can't really save after you defeated Flowey in the neutral run, but instead you have your save file right here and you press on.

So, in a way, the game must have already saved the progress for you (as you defeat Flowey), therefore it does not matter whether you have saved the game at least once or not. Because when you do a no save run, the game knows, and Sans knows. So I don't think that's supposed to happen, or else Toby might be a bit too cruel with the save/load mechanics for achievement hunters.