After Darry hits Pony-boy, he runs into Johnny and asks him for a cigarette. A few minutes later when they are in the park Johnny asks Pony for a light because "I found this butt, man." When Johnny gave Pony the cigarette before, you could see he still had some in the pack.
I don't think this IS a slip-up, as I've been the only one with smokes in a group before, and if you let on that you've got more, you soon won't have them. This was likely more a clever ruse to hang on to some of his smokes.
i dont thing johnny would keep his cigarettes from pony. what i think he found was a really good cigarette or a cigar butt. cause they get the cheapest cigarettes