On the day in question, Severus Snape, who had walked up to the hospital wing to see if Poppy required anything brewed, was utterly unaware that many students were ready and waiting inconspicuously around the Charms corridor where the event was to take place. He left the hospital wing with an armful of filled potion vials, which Poppy had purchased from an apothecary and needed to be brought up to standard, because Severus had been too busy to brew them himself. It was Severus' task to fix them; he was a truly gifted Potions Master and nothing bought from a store could beat his. He set off down the corridor, his mind set on the new task of fixing the potions and ways in which he could improve them.
George, who was waiting near the hospital wing to watch Severus, frantically ran towards Fred, yelling, "CALL IT OFF!" when he realised that the Potions Master was, in fact, not empty-handed but carrying multiple iridescent, and acidic, potions vials. It was too late. Severus had already begun to quickly walk down the corridor. It felt like time stood still. Severus' foot caught on the invisible string; he gasped in shock and put his hands out to break his fall, the potion vials breaking all around him. The string was hung too close to the stairs, and Severus tumbled down the stairs with a surprised yelp. He came to a stop with a sickening thud at the bottom and lay very still.