Slytherin wins both the Quidditch Cup and the House Cup (1989)

“Gryffindor have never gone so long without winning. Slytherin have got the cup six years in a row!”
— Nearly Headless Nick to new Gryffindors (PS7)

“If they won their next match, against Hufflepuff, they would overtake Slytherin in the House Championship for the first time in seven years. ”
— Harry Potter after one of Oliver Wood’s Quidditch practice sessions (PS13).