In 2011, Paul Rudd told Entertainment Weekly that he was literally not sure he ever got paid for this movie - he thinks that because the budget and the production staff were both so incredibly small, they may have just overlooked making out a check for his salary.
During the scene where Beth (Janeane Garofalo) is announcing which campers need to put their trunks out for an early bus, Janeane Garofalo improvised most of the names. You can hear her call for "David Ben-Gurion," who was the first Prime Minister of Israel. Another name she calls out is "Rabbi Menachem Schneerson"; Menachem Mendel Schneerson was perhaps the best-known twentieth-century leader of the Chabad Lubavich Hassids, a movement within Ultra-Orthodox Judaism. For many of the other names, she is reading from Camp Tawonga's 5-year-club plaque.
The shooting script included a dark comedy scene where careless counselor Andy murdered a camper in order to cover up the many drowning or other deaths that occurred due to his negligence, but director/co-writer David Wain cut the scene after he gave the screenplay to his father to look at--because Wain's father said Wain would be disowned by him if he put something that horrible in the movie.
According to the Entertainment Weekly article from the July 1, 2011, issue, actor Hank Azaria was a camper at Camp Towanda from age 6 to 15. Janeane Garofalo (his co-star from 1999's Mystery Men) called him after she saw his name on a bunk plaque. Garofalo then gave him a shout out during a scene where she ad-libs a roll call of campers, using the name Jessica Azaria.
While Coop is listing for Katie all the reasons he loves her, one of the things he says is "I love it that sometimes for no reason you're late for shul." "Shul" is a Yiddish term that refers (sometimes colloquially) to a Jewish house of worship (also known as a synagogue or a temple).
The trivia item below may give away important plot points.
While a guest on "Live with Regis & Kelly" in 2010, Bradley Cooper was asked which of the many actresses he has co-starred with was his favorite on-screen kisser. Instead of an actress, he chose Michael Ian Black, with whom he shared a love scene in Wet Hot American Summer (2001).