Harry and Draco aim curses at one another, but hit bystanders by accident...

Extract from Harry Potter and the Goblet of Fire

by J.K. Rowling

‘Want one, Granger?’ said Malfoy, holding out a badge to Hermione. ‘I’ve got loads. But don’t touch my hand, now. I’ve just washed it, you see, don’t want a Mudblood sliming it up.’
 Some of the anger Harry had been feeling for days and days seemed to burst through a dam in his chest. He had reached for his wand before he’d thought what he was doing. People all around them scrambled out of the way, backing down the corridor.
 ‘Harry!’ Hermione said warningly.
 ‘Go on, then, Potter,’ Malfoy said quietly, drawing out his own wand. ‘Moody’s not here to look after you now – do it, if you’ve got the guts –’
 For a split second, they looked into each other’s eyes, then, at exactly the same time, both acted.
 ‘Furnunculus!’ Harry yelled.
 ‘Densaugeo!’ screamed Malfoy.
 Jets of light shot from both wands, hit each other in mid-air, and ricocheted off at angles – Harry’s hit Goyle in the face, and Malfoy’s hit Hermione. Goyle bellowed and put his hands to his nose, where great ugly boils were springing up – Hermione, whimpering in panic, was clutching her mouth.
 ‘Hermione!’ Ron had hurried forwards to see what was wrong with her.
 Harry turned and saw Ron dragging Hermione’s hand away from her face. It wasn’t a pretty sight. Hermione’s front teeth – already larger than average – were now growing at an alarming rate; she was looking more and more like a beaver as her teeth elongated, past her bottom lip, towards her chin – panic- stricken, she felt them, and let out a terrified cry.
 ‘And what is all this noise about?’ said a soft, deadly voice. Snape had arrived.
 The Slytherins clamoured to give their explanations. Snape pointed a long yellow finger at Malfoy and said, ‘Explain.’
 ‘Potter attacked me, sir –’
 ‘We attacked each other at the same time!’ Harry shouted. ‘– and he hit Goyle – look –’
 Snape examined Goyle, whose face now resembled some- thing that would have been at home in a book on poisonous fungi.  ‘Hospital wing, Goyle,’ Snape said calmly.
 ‘Malfoy got Hermione!’ Ron said. ‘Look!’
 He forced Hermione to show Snape her teeth – she was doing her best to hide them with her hands, though this was difficult as they had now grown down past her collar. Pansy Parkinson and the other Slytherin girls were doubled up with silent giggles, pointing at Hermione from behind Snape’s back.
 Snape looked coldly at Hermione, then said, ‘I see no differ- ence.’
Hermione let out a whimper; her eyes filled with tears, she turned on her heel and ran, ran all the way up the corridor and out of sight.

Harry Potter and the Goblet of Fire

by J.K. Rowling