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.
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.
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.