S01E05, S01E10, S02E05
In S02E05 that Nagisa (Kashiwagi), in h own thoughts, refers to Kaguya as Kaguya-san. Probably if you check the scene again Nagisa will also address Kaguya as Kaguya-san.
In S01E05, Nagisa addresses Kaguya as Kaguya-sama.
In S01E10, Nagisa addresses Kaguya as Shinomiya-san.
Question 1: What's up with this?
Update: (minor spoiler for Chapter 138)
Actually, I checked out the manga and apparently We see that
Nagisa calls Kaguya as Kaguya-chan.
Question 2: Again, what's up with this? For manga past chapter 100, please use spoiler text.