AN: The Slytherin Common Room Ceiling prank is not mine, I read it in a fic a couple of years ago and it's never gone away. If anyone knows from where I've stolen it, please let me know so I can credit it properly.
Pranking Severus
"There you are Severus. Come and join us."
Snape stood in the doorway glaring at Minerva, who sat drinking and laughing with Black, Lupin, the Weasley twins and the Golden Trio. Certainly not company he would choose to keep if he had any say in the matter. Which he did.
"I do apologise Minerva, but I believe I would find more congenial company in Trelawny's tower," he said, greeting most of the room's inhabitants with a sneer. "I just came to inform you that Albus has returned to the castle and that you may remain here at your leisure. Which clearly you had every intention of doing," here there was a more significant sneer at the tumbler of Firewhiskey in the Transfiguration mistress' hand.
"Really Severus, Albus had already mentioned that he would." She seemed to contemplate this. "Ahh, he must, therefore, have had an ulterior motive in sending you with the message," Severus maintained his blank façade in the face of blatant manipulation and message-bearing duties, Black in particular looked positively delighted with how he had been treated by Dumbledore. Minerva was still going on however, "… must surely have been for you to join us." Here he could see the taut expressions of almost all in the room hoping for his refusal, with which he would have been quite happy to please them on this occasion, but Minerva was still talking, "we were just trying to recall some of the greatest pranks ever pulled. The others were split between the Weasley twins' exit and an incident involving copious amounts of fruit and cream occurring in your fifth year I believe. I however have at least two other contenders." She took a genteel sip of her drink and raised an inquisitive eyebrow at Snape.
"As fascinating as the conversation sounds I have rather more important things to take care of this evening than indulging in compiling lists of the most puerile behaviour to have been exhibited at Hogwarts. Good evening." And with an about-turn he had almost cleared the room before hearing Minerva once again.
"Surely you have an opinion on the subject?"
"Dignifying this topic of conversation with anything more than the contempt it deserves is …"
"Then you don't even want to hear my two further contenders" came the interruption from the clearly tipsy cat-woman. Severus was sure he did not, but the knowing tone of voice she was using and the unusual attempt at subtlety in directing the conversation was rather unnerving. Nothing good could come of this; not with the Marauder-wannabes still with a year of school left. He was on the threshold of the door, little could be done to stop him leaving now. Except now he had hesitated a second too long, long enough for four words to hang in the air:
"Slytherin's common room ceiling."
Bemused looks greeted this rather cryptic comment. Shortly followed by gasps from Lupin and Black's corner.
"Pray continue Minerva," said Snape turning once again to face her with a blank expression.
"Do you remember that December when the Slytherin common room was transformed by the spell used in the Great Hall? No longer was the room dark and full of torch shadows but it was enveloped in the soft green of the lower lake water, and with grindylows and the occasional merperson on show." There was a twisted smile on the lined Scotswoman's face.
"Why, how poetic you get when you drink Minerva. I do of course remember as it happened in my very first year, it was quite the event. And you are certainly right in putting it in a higher class of magic than the 'fruit and cream of my fifth year', though calling such re-decorating a 'prank' is a little unfair. Did Albus ever discover who did it?"
"Come now, Severus. Surely you know?"
"Not in the least Minerva. There were several suspects at the time I recall, but all seemed to have rather ironclad alibis. I would personally have suggested Lucius Malfoy though he did seem entirely put out about the whole affair."
Black piped up with "I heard it was Steggins", and though a staring contest appeared to have started between the two professors it did not stop Snape putting down such a suggestion.
"It could not have been Steggins you imbecile, as, though he seemed to have no reasonable alibi, his magic was pathetic at the best of times and his intellect and creativity were no bigger than yours."
"It was you Severus. I know it was."
At this Snape made a mock bow to McGongall and said with a smile, "My thanks for your unintended flattery of my magical prowess at eleven years old. However, it was not me."
"You've never taken it down."
"No, I happen to like it. But it was not taken down by Albus, Slughorn or yourself if you were so offended by it. It stayed that way throughout my student years and the years before I came back to teach."
"All of the sixth and seventh years either could not have magically done it or had perfect alibis."
"And I had been at school for less than four months."
"The book from which the spell was taken was removed from the library as soon as you started teaching. Don't think I didn't notice."
"That book should clearly have been removed long before. I found it in my later student years, it was obviously full of temptations to able wizards and witches."
"Where were you when it happened?"
Snape snorted. "Really Minerva, it was not me, the notion is ridiculous. I cannot recall where I was since it was over twenty years ago and, even if it was me and you were to prove it so, what would you do? Give me detention?" There were half-grins from Potter and the Weasleys as they tried to imagine this rather bizarre scenario.
"I know it was you since Albus has never said it wasn't." Raised eyebrows met this apparent lack of logic. "In any case, there is another incident which I know very well you were the author of. Do you happen to remember a certain interestingly coloured meal?"
At this Lupin and Black started laughing uproariously and Snape snorted amusedly.
"That was definitely you Snape, I remember James being Gryfinddor enough to get up and shake your hand for it," said Black.
"What was that then?" asked George Weasley.
"It was the funniest thing ever," supplied Lupin. "One dinner, I think it was fourth year, we had all started eating when suddenly the teachers all began to go the strangest colours. First Albus' hair turned purple, then Minerva's went golden, Degson's went a horrible sickly green and Slughorn's became bright pink. Every single teacher at the head table had their hair change colour. The Great Hall went beserk … and … and," Lupin could hardly continue with the tears streaming down his face.
"That prat of a Defence teacher ran out screaming because he thought that we were being invaded by Australian Bangalons," Black managed to finish.
"You did that?" asked Fred with what seemed close to awe.
"No, I most certainly did not," returned Snape with some anger behind the words.
"Oh, come on Snivellus, it was clearly a potion. How did you do it? Bribe the house elves?"
"Do try not to be so ludicrous mutt-lover, it was impossible for it to have been me. And house elves cannot be bribed you fool."
"Severus, you even tightened the security around meals when you started teaching!"
"Simply because I did not want the same thing happening again. You might also consider that it would have been laughably easy for anyone to poison the Headmaster at a time when the Dark Lord was reaching the peak of his power."
"Look, Severus," reasoned Lupin, "it doesn't matter now; just tell us all how you did it."
"Yeah, you git, it was the one and only day that we all thought you weren't quite such a slimy snake after all."
Severus was now scowling uncontrollably at the pathetic mongrel and his companions. The Marauder Wannabes however seemed to be leaning in intently to hear further details of a prank to which no one would own up. "Why couldn't it have been you Professor Snape?" asked Hermione, innocently but fully intent on solving the mystery.
"I will not be interrogated by irresponsible rule-breaking students and their equally pathetic role models," said Snape with quite some disdain while doing another impressive turn to stalk out of the room. He walked straight into Mad-Eye Moody.
"Hey boy, watch where you're going," scowled the battered old Auror. Snape snarled at Moody and pushed past him. He slammed the front door on his way out, awakening Mrs Black and letting her wreak havoc on the Gryffindors' ears.