DISCLAIMER: All of Harry Potter is JKR's. I play in her sandbox. She is the queen. All hail Queen Rowling. Yeah.
Bits of the following were copied verbatim from Harry Potter and the Philosopher's/Sorcerer's Stone, pages 207-214.
"You asked me to come directly to you, Professor, if anyone was wandering around at night, and somebody's been in the library—Restricted Section."
Severus raised an eyebrow. "The Restricted Section? Well, they can't be far, we'll catch them." Severus started down the nearby corridor, Filch just behind him. Something moved on the floor just out of his vision, but when he looked directly at it, it was still. The door at the end of the corridor was slightly ajar, and it moved almost imperceptibly. Severus knew what lay behind that door. Whatever idiot finds himself in that room by accident will be caught by someone else, he thought, turning on his heel and striding away, still followed by the caretaker.
"I think," Severus said as they rounded the corner toward the library, "that I will not be assigning detention, unfortunately."
Filch nodded and ambled off down a different corridor. When Severus was convinced the caretaker had gone, he swept back to the room he'd intended to avoid, wordlessly making himself invisible and stepping into the room silently.
Severus could have Apparated with half of Slytherin House into the room and he was sure he would not have attracted the attention of the dark-haired, bespectacled boy who stood before the strange mirror. Still, Severus remained silent and invisible.
"Mum?" Potter asked the mirror incredulously. "Dad?" The boy looked all round the mirror; it didn't take a Legilimens to know what the Boy Who Lived saw in the Mirror of Erised. The Potter boy, who knew nothing about his family, saw the lot of them, Potters and Evanses alike, crowded into the mirror.
The boy's face and hands were pressed against the glass as if he could not get close enough. Idiot boy, Severus thought bitterly. They can't touch you. Severus knew he could quite easily give Potter detention, and he would very much have liked to, but he couldn't seem to bring himself to it.
Severus stood a few feet behind Potter, but saw only his reflection, and the boy's. He was both relieved and infuriated that this ignorant boy stood in front of the mirror, in Severus' path. After a few long minutes, Severus strode out of the room, deliberately making noise out in the corridors as he walked.
Severus found Potter and the Weasley boy in the mirror room the following night, arguing loudly, and was pleased to see Mrs. Norris strutting into the room. The door opened and shut and Severus heard shuffling of feet, amazed that the boys didn't notice their terrifying Professor standing in the shadows.
The second night after Potter's discovery of the mirror, Severus stood in front of the mirror as he had done many times since he'd discovered it himself. He only allowed himself to come sparingly, but tonight he absolutely needed to see her. He had come earlier than usual, as he expected Potter to show up again, despite having overheard Weasley urging him to stay away.
Lily, he thought the moment she appeared. She always appeared like this, walking slowly toward him out of the depths of the mirror. His reflection was dressed differently, more formally, and he was younger. She was as he remembered her; young and happy. She wore a flowing white gown resplendent with lace, and carried a bunch of white daylilies. Her smiling face was covered by a thin lace veil that cascaded down her back, trailing behind her as she walked toward him. She reached a hand toward him and he tried to resist stretching his toward her, but he had never been able to keep his hand by his side, and tonight was no different. He touched the glass with his fingertips, and a tear fell down her face, though she was still smiling.
Footsteps sounded in the hall, and Lily vanished as Severus stepped away from the mirror, disappearing into the shadows. Potter ran in and sat in front of the glass exactly as Severus had expected.
"So—back again, Harry?" came a familiar voice.
"I–I didn't see you, sir," Potter stammered.
Nor did I, Severus added silently.
"Strange how nearsighted being invisible can make you," said Albus, smiling his irritatingly gentle smile. He slid off the desk to sit next to Potter. "So, you, like hundreds before you—" Albus' eyes met Severus' invisible ones for an instant "—have discovered the delights of the Mirror of Erised."
"I didn't know it was called that, sir," Potter replied. Fool, Severus thought, looking at the inscription above the mirror.
"But I expect you've realised by now what it does?"
"It—well—it shows me my family—" You're not the only one who sees them...
"And it showed your friend Ron himself as head boy."
Potter was much more surprised than Severus was to learn that Dumbledore had been in the room the previous night. Severus was willing to bet that Dumbledore had been there the night before that as well.
"Let me explain," Albus was saying. "The happiest man on earth would be able to use the Mirror of Erised like a normal mirror, that is, he would look into it and see himself exactly as he is. Does that help?"
Potter took half an aeon to respond, and Severus wasn't surprised that he'd missed the mark again. As Albus explained further, Severus read the inscription again: Erised stra ehru oyt ube cafru oyt on wohsi. Since he had discovered the mirror, Severus had tried to figure out what the words meant, what language they were from. He had even asked Albus, after he'd had an encounter appallingly similar to this one, but Albus had merely smiled and smoothly changed the subject. They weren't in any language Severus could find, though he had looked for years.
Albus change of tone caught Severus' attention again. "... will be moved to a new home tomorrow, Harry, and I ask you not to go looking for it again." Ah, yes, Severus thought with some irritation. The corridor with the Cerberus... "If you ever do run across it, you will now be prepared." Severus nearly choked at Albus' lack of subtlety, but sobered when Albus said, "It does not do to dwell on dreams and forget to live, remember that. Now, why don't you put that admirable cloak back on and get off to bed?"
Severus knew the advice wasn't really directed at the boy, who was now standing. "Sir—Professor Dumbledore? Can I ask you something?"
"Obviously, you've just done so. You may ask me one more thing, however."
"What do you see when you look in the mirror?"
Severus could have hit the boy for such a question, but Albus merely answered, "I? I see myself holding a pair of thick, woollen socks." When Potter only gaped, Albus continued, "One can never have enough socks. Another Christmas gone and I didn't get a single pair. People will insist on giving me books."
The boy seemed satisfied, but Severus guessed Albus wasn't being entirely truthful. But then, it was a very private question, one which Severus himself would have answered by giving the boy a week of detentions full of scrubbing the dungeons with a toothbrush. When the boy had gone, Severus saw Albus glance at the mirror, and he thought he saw a flicker of longing cross the elder man's face.
Severus read the inscription again. It shows us nothing more or less than the deepest, most desperate desires of our hearts, Albus had said.
Desire, thought Severus, looking at the words. Suddenly, the inscription made sense. Severus kicked himself for not seeing it sooner.
"Erised," Albus said. "Such a strange and dangerous place. Goodnight, Severus."
Albus disappeared, leaving Severus in front of the mirror. Lily smiled from behind her lace veil and stretched out her hand to him.
I show not your face but your heart's desire.
A/N: I keep coming up with ideas for this collection. If I ever outright defy canon, you may tear me to pieces, just so long as you use references as your weapons.