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 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.
Chosen answer: Simple question, simple answer. Throughout the movie, and on the way to the rumble, he wears a (different colored) plaid shirt, white tee, blue jeans (on the day of the rumble he asks Darry "and where are my jeans?), and worn out tennis shoes.