Revealing mistake: When Ponyboy and Dally visit Johnny at the hospital to tell him the Greasers won the rumble, Johnny dies in front of them. If you look closely in one shot, you can see slight breathing (rising and falling) under the cot blankets.
Continuity mistake: In the scene outside the movie drive-in, when Ponyboy, Johnny, and two girls are confronted, if you look behind Randy, Marcia gets in the Mustang twice.
Continuity mistake: In the scene outside the theater where Bob and Randy confront Ponyboy, Two-Bit and Johnny for being with Cherry and Marcia, if you look behind them, Marcia gets in the Mustang twice.
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.