The sources are pretty numerous
For example: http://steptos.com/olympics/where-were-the-summer-olympic-games-in-1952
"Olympic Games 1952 in Helsinki, famous for one fun fact. They entered the history of the Olympic Movement as a game that has not been closed. August 3 at the closing ceremony of the games IOC President Sigfrid Engström said oration, but forgot to say the final phrase ascribed Charter: "I declare the XV Olympic Games closed." Olympics in Helsinki lasted two weeks, but was not completed until now".