"It’s warmer than I thought."
That scene always gets me. The last time Anna was in the same room as Elsa, it was snowing in there. She couldn’t remember that night exactly, since the trolls took away her memories from then, but even when you don’t recall something that happened in a room, you remember the atmosphere that was there. Anna had remembered “Oh Elsa is here, everytime we were in a room together when we were kids it was always freezing!” So Anna was surprised by how hot it was in that room. Elsa notices the vague remembrance and quickly changes the subject with “What is that amazing smell?”