I never thought this was a mistake at all

My assumption was Mufasa yelled "Scar" and it was infact Scar who replied with that faint "Brother" as in to make a point that he is his brother...an heir to the throne, its like like scar is asking for some family respect and recognition after being ignored all this time because he is not the king and now he is Mufasa's only way out to survive. Out of desperation Mufasa then calls him "Brother", because listening to all the dialog of the film up until that that moment, Mufasa never once calls him brother.

If you hear closely that faint "brother" does sound a lot like Scar.