One day after class exams, Sirius Black was feeling bored. With a devious smirk, James Potter noticed that Severus Snape was standing outside. The pair loved to tease and harass Snape so Potter knew that it could lift Black's spirits.
Hey look! I know what will cheer you up. Little Snivellus is outside!
Snape was outside minding his own business. More than likely daydreaming about a future life with Lily Evans.
When Snape saw the two boys approaching him, he took out his wand. Unfortunately for him, Potter cast the Disarming Charm and he was left without his wand.
Expelliarmus
Lily appeared and in that short distraction time, Snape was able to pick up his wand again. He quickly cast Sectumsempra at Potter but ended up hitting both boys with the spell.
Sectumsempra
Lily looked on in horror as James and Sirius were suspended in the air upside down. She went to reprimand Severus but out of his own frustration, he yelled back at her...calling her the worst thing you could call someone in the wizarding world.