No. Not a request. This is just a random idea that came into my head cuz I always freak out over this one thing (I'm not gonna spoil it, cuz it'll give away the oneshot).
And yes, the title is a pun. You'll see what it means later in the oneshot. ;3
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Sans stared out the car window, the volume of his music so loud through his headphones that it made his skull vibrate. But that was the least of his concerns.
Dear god, he hoped Papyrus wouldn't bring it up. Talking about it didn't make him feel any better. In fact, it only made it worse.
Crystel had told him that it wasn't a big deal. But she didn't know the pain of it all. Of what he'd been through. His memories of his childhood, being experimented on in the lab.
"We're here Sans," Papyrus announced, tapping Sans on the shoulder.
Sans jumped, turning down the volume of his music. "Huh?"
"We're here," Papyrus repeated.
Sans felt his soul skip a beat. But he pushed down his fear and took off his headphones before getting out of the car.
He looked up at the doctor's office, his soul beating faster as he followed Papyrus into the building. He sat down in the waiting room, trying to keep as cool as he could.
After a few moments, Papyrus came and sat down next to him. The two brothers didn't say anything.
Before long, Sans heard his name being called by one of the nurses. His soul pounded as he stood up and started walking over to the back room.
He stopped as he felt a hand on his shoulder. He turned around to see none other than Papyrus. "Do you want me to go back with you?"
Sans smiled, the pinpricks of light in his eye sockets dull from fear. "Yeah. That'll make me feel somewhat better about this.."
Both of the skelebros followed the nurse back into one of the examination rooms. Sans toned out the sound of her voice as she told him the inevitable.
He was getting a flu shot. And there wasn't anything that could be done about it.
He felt a hand on his shoulder, and he looked up to his his brother. "You going to be okay with this?"
Sans sighed. "I'll be honest with ya bro, I'm pretty freaked out right now."
"It's okay. I'll be here to comfort you. That's what brothers do!" Papyrus replied.
The light in Sans's eyes grew dull. "Bro? Can you promise me something?"
Papyrus looked surprised. "Yes?"
Sans took a deep breath. "If something... anything... goes wrong at all, promise me you'll stop me from hurting anyone."
A look of confusion crossed Papyrus's face. "Why would-"
He was cut off by a knock on the door. "Come in," Papyrus answered.
Sans's non-existent stomach lurched as a nurse walked in with a syringe on a tray in her hand. He clung onto Papyrus, shaking uncontrollably.
"I can't do it... I can't do it..." he mumbled.
"Sans, it's okay. It'll only hurt for a few seconds."
"No! It's gonna hurt a lot more than that! I know it!" Sans sobbed, not wanting to listen to common sense at all at the moment.
Papyrus sat down in one of the chairs and set Sans down on his lap. He allowed his brother to cling onto him as the nurse came over with the needle.
Sans shuddered violently, vivid memories of times in the lab coming back to him. He felt a sharp pain in his left arm for a split second as the nurse gave the injection.
His breath rattled in his ribcage and his left eye flared as the memory of the burning pain from the determination that he'd been injected with so many times as a kid overtook his mind. All he could think about was how much it hurt.
Then he realized; this was way more than just a memory. The burning pain in his chest wasn't in his head. It was real.
   He was losing awareness of the world around him. He could feel control slipping from his grasp. He was vaguely aware of Papyrus trying to comfort him, but it did nothing to snap him out of his panic.
Sans clung fiercely to his brother, still shuddering uncontrollably as he dug the tips of his fingers into the red fabric of Papyrus's scarf. He flinched as Papyrus gently rubbed his shoulder, his breathing growing more ragged with each sharp breath he took.
"Papyrus- NGH! D-Don't let me- URGH! D-Don't let me hurt anyone..." Sans choked out between increasingly loud cries of pain and shallow, uneven breaths.
"Sans! No! You need to calm down! You can't do this right now!" Papyrus exclaimed, holding his brother tighter, despite the sickening sound of bones creaking and snapping that arose from his brother's shivering form.
"I-I cAN'T-" Sans broke off with a groan of agony, arching his spine in response to the pain that wracked his body.
"Yes you can. You just need to fight it. Remember who is in control. But you can't turn now," Papyrus insisted.
Sans moaned, his grip on Papyrus's scarf tightening. Papyrus tried to pry his brother's hands away from his scarf, finally getting him to let go, only to have Sans start clinging to his arms instead. Papyrus winced and choked down a cry of pain as he felt Sans's fingertips dig into his forearm.
Just by the feeling alone, Papyrus could already tell that the transformation was beginning to take hold of Sans. The sharp pain he felt was most likely caused by the claws that were just beginning to form on the tips of Sans's fingers.
"Sans please... You have to fight it... It's not safe to change here..." Papyrus pleaded quietly.
A ridge began to form along the back of Sans's jacket as spikes emerged along the smaller skeleton's vertebrae, pushing against the fabric of his jacket and causing it to stretch.
It wasn't long before the fabric tore completely, revealing the massive spikes that had formed along his spine as they continued to grow painfully from his vertebrae.
Sans looked up at his brother, a pleading look in his eyes. Papyrus shuddered as he realized that the entire left side of Sans's skull was engulfed in icy-blue flames. "B-Bro please... I c-can't fight it forever..."
"Sans, what are you-" Papyrus was cut off as Sans wailed in agony, throwing his head back as his left eye flared blue.
"I-I don't have time! G-Get everyone outta h-here!" Sans ordered, a desperate gleam in his eyes.
"Absolutely not! I will not leave you!" Papyrus insisted, a determined tone in his voice.
Sans grunted in pain, clutching a hand to his sternum. Papyrus could barely hear the sound of fabric slowly stretching and tearing over the sound of bones snapping and magic crackling through his brother's body.
Sans pulled away from Papyrus, stumbling and falling as soon as his feet touched the ground, groaning as pain shot through his legs. He felt a wave of nausea come over him as his feet began to elongate and the tips of his toes began to grow sharper, tearing through the fabric of his slippers.
   He could feel his joints cracking back on themselves as his leg structure began to change into that of a quadruped. The pain that tore through him forced a cry of agony out from between his clenched teeth, which had grown into sharp, curved fangs.
   Sans felt his non-existent stomach lurch at the sound of his own voice. It had deepened and it had a frightening animalistic tone to it. It wasn't even recognizable as his own...
   He didn't have much time to think about it as another wave of pain was sent through his body, causing him to cry out once more. He became aware of an all-too-familiar tingling sensation at the base of his spine as a tail began to grow from the lower section of his spine.
   Sans was aware of his brother staring at him, a concerned look in his eye sockets. He could hear faint screaming from somewhere else in the building, but it was drowned out by the sound of his own distressed wails, which at this point sounded more like incomprehensible roaring rather than actual cries of pain.
   He could feel his face beginning to elongate as horns began to emerge along the back of his skull. He screamed as his face split from his nose socket to his browline, sending a jolt of sharp pain through his skull.
   He panted heavily as the lingering traces of the transformation began to fade. The pain had left him for the most part, leaving only a dull ache. The icy-blue flames that had engulfed his body in the midst of the transformation slowly vanished.
   "S-Sans?"
   Sans turned his head to look at his brother. "P-Papyrusssss...."
   "You scared the living daylights out of that nurse! What is wrong with you?!"
   "Sssorrrryyyy..." Sans rumbled, looking away.
   Papyrus sighed. "It's okay brother. I know. You were just scared and you couldn't control it. You should not be at fault for that."
   "Did I hhurrtt anyone?"
   Papyrus rubbed his forearm, wincing as the fabric of his glove brushed against the deep gouges on his arm from his brother. "No. You may have scratched my arm a little, but it's not bad."
   There was a clang on the other end of the room, causing Sans's head to jerk up. A terrified nurse stood, frozen in fear at the sight of the massive skeletal beast. A metal tray lay upside down on the ground, the empty syringe on the floor a few feet from it.
   At the sight of the object, Sans let out a roar of fury, a feral gleam in his eyes. The nurse shrieked and ran out of the room, eyes wide with terror.
"Sans, stop!" Papyrus shouted, racing towards his crazed brother.
Sans let out another roar, a few wisps of blue flame escaping from his maw. Papyrus threw his arms around one of Sans's forelegs, trying to make him remember who he was.
But it was no use. Sans flung Papyrus off as though he were nothing, continuing to roar and growl as his piercing blue gaze darted around the room, searching for the threat he still thought was there.
Sans's tail flicked slightly as he realized that nothing was there.
Must... Find... Threat...
He shook his head, not wanting to listen but not having much of a choice anyway. He felt a distinct burning sensation in the back of his throat as magic built up in his open jaws. He turned to face the open doorway, his eyes narrowed in fierce concentration.
If he couldn't get through the door as it was now... Then he'd just have to make it so that he could.
"Sans, no! Stop it!" Papyrus protested, activating his magic and holding his brother down.
A beam of raw magic burst from Sans's maw, going straight through the doorway and destroying everything in its path. Sans roared in protest, struggling to escape the effects of Papyrus's magic.
"You need to stop! Calm down! Please..."
Sans didn't seem to hear him. He just continued to thrash as he roared in fury. His tail smashed against the floor, barely missing Papyrus.
The taller skeleton dodged swiftly, darting forward to throw his arms around his brother's neck. Sans growled, trying to pull away but having no luck.
"Shh. Calm down. You're not in danger. I'm not in danger. Everything is alright," Papyrus murmured softly.
Sans's growling grew quieter, his gaze softening. His struggling weakened and he leaned into his brother's embrace.
"That's right. There's no threat here. You are safe now," Papyrus continued, stroking the side of Sans's skull gently.
Sans's growling gradually turned into a contented purring as he closed his eyes, the feral part of his mind giving way to his normal thoughts as he regained control.

A/N: Yay! Finally got this done! XD
I actually wanted to get this done yesterday, but I sadly couldn't. :/
Oh well, it's done now! :3
Hope you all enjoyed! I'll try to update a bit more now that I'm off from school for two weeks! :P
Stay determined!
-Sora

Edit: 4k reads?! Gosh, I really didn't think this would be that popular! Thank you guys so much! :3