Slytherin wins both the Quidditch Cup and the House Cup (1987)

Nearly Headless Nick tells the new Gryffindors about the House Cup, saying that

“Gryffindor have never gone so long without winning. Slytherin have got the cup six years in a row!” (PS7)

Having been pushed hard by Oliver Wood in Quidditch practice, Harry thinks

“If they won their next match, against Hufflepuff, they would overtake Slytherin in the House Championship for the first time in seven years. ” (PS13).