"Fred and George had made sure nobody was likely to forget them too soon. For one thing, they had not left instructions on how to remove the swamp that now filled the corridor on the fifth floor of the east wing."
The fifth floor corridor of the east wing at Hogwarts School of Witchcraft and Wizardry is mentioned in the book as the location where Fred and George Weasley create a Portable Swamp as a means of annoying Dolores Umbridge and Argus Filch.
The Weasley twins, who left Hogwarts to start their joke shop in Diagon Alley, Weasleys' Wizard Wheezes, left the swamp without instructions on how to remove it. Umbridge and Filch couldn't remove it after trying many things, so the area was roped off. Filch was given the annoying task of "punting students across it to their classrooms" (OP30). When Umbridge was sacked as Headmistress at Hogwarts, Professor Filius Flitwich easily removed the swamp, though he left a small patch of it in the corridor. He claims to have done it because it "was a good bit of magic" (OP30), though Ron Weasley thought it was a tribute to the twins.