Revealing mistake: When Two-bit Mathews returns from the hospital gift shop with a copy of "Gone With The Wind", he hands the book to the nurse asking her to make sure that Johnny gets it. The book he hands her is an obviously used paperback with the upper front top right corner and pages all bent back. Shouldn't a gift shop sell NEW items? The book is only shown from the non-title side so they could have used any paperback for the prop as long as it looked like it wasn't several decades old.
Continuity mistake: In the opening scene, in the long shot, Dallas is leaning back against a lamppost with his knee bent and foot against the post, trying to light a cigarette. Cut to the close up, he's still trying to light the cigarette, but he's leaning in the pole with his shoulder.
Answer: Johnny is recalling the argument he heard his parents having; the voices were meant to sound angry but not to be understood. Its to help us understand his horrid home life.