Board Thread:General Discussion/@comment-35350498-20181110193207/@comment-37549604-20181118152215

One more thing.

Sans clearly knows that each timeline eventually resets.

What if in one timeline, everyone got to the surface and the events of deltarune happened, and then it got reset again, but sans somehow kept a souvenir?

This would explain why that picture is found in undertale, which seems to be separate from, or chronologically before deltarune. The timeline got reset, and sans kept a picture from the future. that's also why he's reminding himself not to forget.