It happened in Ron Weasley's Sixth Year, at the very start, when he was hexed by Draco Malfoy to fall down the stairs and broke his left wrist. They normally waited at least a month into the school year before wounding each other seriously enough to warrant a trip to the Hospital Wing, but this was the year that war had officially been declared and tensions erupted early.
Ron lurched his way to Madam Pomfrey's care, slightly shocky from the pain. She tutted when she saw his pale face and directed him to one of the beds. "By the time you've done healing, it'll be past curfew," she warned. "You'll have to stay overnight."
Exhaustion was already dragging Ron's eyelids down, so he nodded and yawned simultaneously, then settled into the thin mattress with its scratchy blankets. There was a reason why he hated to stay overnight in the Hospital Wing; they always tended to give him the wonky beds. Everyone else he knew of that managed to land in there always got a cushiony mattress and fluffy pillows – Ron got no pillows at all.
Hours later, Ron opened his eyes again. He would never be entirely sure of what had woken him: a shift in the shadows, the murmurs of two soft voices, or maybe even the encroaching sense of 'This Is Destiny Approaching, Pay Attention!' Once he was awake, what he did see was a tall shadow of a man holding a still boy in his arms, and the familiar form of Albus Dumbledore trailing after. Madam Pomfrey could be heard in her office, muttering to herself and gathering potions.
The tall man laid down his burden onto a bed in the far side of the room, one that had curtains to pull around it. Ron could hear a sigh, and then a barely audible whisper, "He'll be alright. He's had worse; he's more exhausted than anything else."
Dumbledore's voice was similarly quiet in response. "This would not have happened if he had been at Hogwarts." The accusation was simple and straightforward.
The man nodded. "Yeah, figured that after last year… but I'd hoped that if I could put up more wards…. Well." Despite being a whisper, the man's voice took on a steely tone. "You'll have your desire now, Albus. I leave him to your care while I hunt the traitor down." He placed one long-fingered hand on the head of the boy he had carried, then let go and turned to leave.
"You won't even wait for him to wake?"
The man paused. "He knows what must be done. I've raised him well – despite all your misgivings. I'll be in touch… and I'll be back to retrieve my godson."
Dumbledore said, simply, "Be safe, Sirius."
It seemed to Ron that he watched Dumbledore lean over the boy for a long time after that, and then there was sleep once more.