My Big Cool Prediction: The Shadow Crystals are showing us the game's timeline backwards
The Shadow Crystals, in the Light World as glass, are showing us the game's timeline backwards. This will be revealed in Chapter 5 with the last Shadow Crystal, where it shows something from Chapter 2 or 3 instead of something from the future. Because of this, we can reference the previous 3 crystals as reveals for Chapter 7, 6, and 5 right now.
1st: See through hand = Kris loses hand in Chapter 7 2nd: Angry Susie = Susie finds out about the double-crossing in Chapter 6 3rd: (unused) Frozen Undyne = Possibly only showing us Weird Route stuff, revealing the Chapter 5 Sacrifice. 4th: Noelle Whispering = Kris does one-on-one talk to Noelle in Chapter 4, in the Weird Route.
Given that the glass only shows us stuff in specific locations/circumstances in Chapters 2 & 4, I wouldn't be surprised if we only get this reveal in a specific light world location in Chapter 5. If it shows us something like " * Susie & You sleeping on the couch" it could only be revealed at the end of the chapter. Even if we start there, if it's day time it might not "work" because you don't sleep during the day.