Slytherin wins both the Quidditch Cup and the House Cup (1988)

Harry Potter is told during his first year by Nearly Headless Nick that Slytherin House has won the House Cup for several years:

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

Later he 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).