The detail that Ponyboy isn’t wearing a coat that night.
All throughout that fateful night and its many cataclysmic events - the drive in, the lot, the slap, the jumping, the stabbing, running to Dally - there’s the through-line that it’s cold outside and Ponyboy didn’t think to wear a jacket, so he’s walking around freezing in a sleeveless sweatshirt the entire night.
He’s exposed, unprotected, unprepared. He’s in over his head.
He’s a child. He hasn’t mastered the basics of taking care of himself. Forgetting to wear a coat when it’s cold is classic shorthand for “child.” He’s about to go through things that would break an adult, and he’s just a child who can’t even remember to wear a coat when it’s cold outside.
He tries to look tougher than he is, like he doesn’t need protection. He tries to look invincible but he just looks vulnerable; in fact all the more vulnerable for trying so hard to look tough. Everyone tries to tell him that he’s not taking care of himself properly, but he won’t listen.
The way a different character, specifically a different member of his family/his gang, comments on it at every juncture. At each point in the night, at each different location - first Darry at home, then Johnny in the park, then Dally at Buck’s - all comment on Pony’s insufficient outfit. A demonstration of how Ponyboy brings out everyone’s big brother instincts. Everyone speaking as portents of fate, like the three witches in The Scottish Play, prophetically warning the protagonist of cold and “freezing to death” and “pneumonia”: of the looming danger and death and illness and tragedy.
It makes his circumstances that much worse. Sleeping in the lot, running out after Darry slaps him, getting drowned in the fountain, he’s that much colder.
It’s a sweatshirt with the sleeves cut off. It’s a shirt that was made with sleeves, that’s supposed to have sleeves, but doesn’t. They don’t have the things they’re supposed to have. Money. Safety. Parents. Anything they do have is defined by the absence of its complete function. Whatever resources they have are sparse, ripped up, insufficient imitations of what they’re supposed to be.
It’s a background detail of discomfort throughout the whole saga, seemingly mundane but vital. It doesn’t seem as important as almost drowning, but it exacerbates it. The discomfort the hardship of their environment is always an undercurrent of their more dramatic problems.
It’s Darry’s care and lack of care at the same time. Darry scolds Ponyboy for not wearing a coat, but then hits him which is what drives Ponyboy to run out of the house into the cold, but the cold is part of what makes Ponyboy want to go back home despite everything. It’s all the ways Pony needs Darry to take care of him but doesn’t want it; wants it but won’t admit it. It’s all the ways Darry takes care of him, and all the ways he doesn’t. All the ways Darry tries and all the ways he fails.
It’s Soda’s sweatshirt. It’s a hand me down, as is probably the youngest brother’s entire wardrobe. It’s clothing that’s not his, that he’s tried to customize for himself only to have it end up worse, less suited for its intended purpose. It’s a classic greaser piece of clothing - a ripped up DIY hand-me-down - and Ponyboy isn’t quite suited for Soda’s full fledged greaser uniform just as he’s not quite suited for Soda’s full fledged greaser lifestyle.
It paves the way for Dally to give Pony his jacket at the end of the night, but at the same time doesn’t really, because even if Pony was wearing a jacket, even if it kept him warm all night, it would have ended up soaking wet and useless after the fountain. No matter what superficial protection he has - a jacket, a blade, a busted bottle, a companion - he would end up a target by virtue of who he is and the world he lives in.
It’s the first time Pony notices Dally performing an act of kindness that surprises him. It’s the first time Pony notices Dally’s similarities to Darry as Dally takes on a parallel big brother role: “‘you ought to know better than to run away in just a sweat shirt, and a wet one at that. Don't you ever use your head?’ He sounded so much like Darry that I stared at him.” It’s the parallels between Darry and Dally, the two older characters who seem cold and hard because Pony can’t see the good in them just yet.
It’s Ponyboy’s vulnerability. The image of a kid walking around in a sleeveless sweatshirt shivering in the cold, evokes vulnerability. It makes you want to wrap this poor kid up in a hug, usher him indoors, make him change into some warm clothes, wrap him in blankets, and make him tea. But instead you watch what happens to him instead. You watch as he’s slapped by his brother and guardian, jumped, almost drowned, and sent on the run for murder, and all the while he’s not wearing a coat and it’s dark and he’s soaking wet and his sweatshirt is sleeveless so there’s no barrier between his skin and the blood of a dead boy, and he is so, so cold.
