A/N: It's been a while since I got inspired to do something. Hopefully, this turned out as well as I hope it did. As always, I don't own the Potterverse.
Slytherin House was not notable in Hogwarts history for its prankster tradition. It was not that the house did not contain this particular brand of student, but those who did convene under this banner did so quietly (save for the all out war that the Marauders drew them into, but that is a story for another time). Until the Mad Slytherin became the terror of Hogwarts for the 1996-1997 school year.
The prank that would propel the Mad Slytherin into future editions of Hogwarts: A History was, on its face, surprisingly mundane. It consisted of placing a giant stone in front of the entrance to Ravenclaw Tower emblazoned with a massive green snake and Slytherin painted in impeccable cursive writing on it.
Seems fairly easy, doesn't it? A simple Shrinking Charm, the proper placement, a Finite, a bit of paint, and voila, you're done. The average 3rd year could probably give it a go. Which would certainly be true, if that's how it had actually been done.
In reality, the prank was far more ambitious and cunning than it was first thought to be. For starters, there was no rock. Many Ravenclaws (primarily girls in 5th, 6th, and 7th years) will still swear to you this day that they felt the rock, cast spells at the rock, and one who even dislocated a finger in her hand throwing a punch at the rock. None of them will even entertain the notion that they were Confunded to believe a rock was there. (As for the girl who dislocated her finger, well, you try hitting a stone bust of an eagle with your fist and not damage something in the process. The Ravenclaw guardian still has a slight imprint of a knuckle in it to this day.)
Secondly, ask anyone else who was able to get a glimpse at the "rock", and you'll get the same response: "Of course there wasn't a rock there!" Ask the expert who was called in about the situation, and he'll tell you that a clever runic array was etched on the pedestal that the Ravenclaw guardian rests upon, hidden by the eagle itself. While their usage was "unorthodox", in the expert's opinion, he found them to be mostly harmless, at best having no action, and at worst creating an illusion of a large stone in the minds of those whom the runes identified as "tending towards thievery, malicious intent, and wishing ill-will upon another."
The school was never able to come to any sort of conclusion as to who put the array in place (or how it happened with no witnesses), but since the illusion made a strong reference towards Slytherin House, the incident was chalked up to the Mad Slytherin and left alone for lack of other compelling evidence.
Other than the Mad Slytherin's hijinks, the 1996-1997 school year also found itself in Hogwarts: A History as being a year where inter-house cooperation rose to near historic levels. Most prominent among these was the friendship between 5th year Ravenclaw Luna Lovegood and 6th year Slytherin Daphne Greengrass, who could often be found in the library looking over notes and references for their Ancient Runes classes. At least, that's what they told everyone they were looking at.
Curiously, the summer before that school year, a large stone had appeared in the backyard of the Lovegood home. Though few people were ever out that way to see it, they would certainly be able to identify it by one of the two painted sides it had. The one facing away from the house had a giant purple raven on it with Ravenclaw painted on it in very neat script. The reverse side….well, you can probably guess what it looks like.
A few years afterward, a new adornment topped the stone: a painted gold band with a diamond attached. If one looked closely, the band was actually numerous instances of "yes" in a similar script as the Slytherin side of the stone, and the diamond the word "forever" looking much like that of the Ravenclaw side.