The following is summarised from an interview with Brandon Sanderson in 2010:
Q: Were there parts that you were like, just wow, this is just really weird having to write this?
A: The other scene has not come up yet so I can't tell you what it is. It was just a "wow." It was kind of that, "How did I miss that?" in part, and also a "I really need to make this really work really well." And anyway, I can tell you about that next year.
Does anybody know which scene Sanderson was talking about?