Several years ago, four Ravenclaws managed to brew the famous Polyjuice Potion around the time students would take their O.W.L.S. Attempting some laughs, they decided to snag a hair from Professor Snape during class and added it to the cauldron. They then mixed a minuscule amount into everyone's pumpkin juice before breakfast, and the Great Gall was soon full with Severus Snape's, dying of laughter.
The Four Ravenclaws were never caught
