Continuity mistake: In the scene at the cove where Suzy and Sam are talking after swimming, the flower stem over Sam's ear keeps changing positions. In some shots it's parallel to the temple on his eyeglasses, and in others it's drooping down over his cheek.
Continuity mistake: Suzy flashes white panties while getting into the tent, but is wearing red check panties later.
Continuity mistake: In the scene where Scoutmaster Ward wakes up to an empty Camp Ivanhoe, he is without his neckerchief at the ironing board; then he walks through the deserted camp and the neckerchief is on. In the next scene, as he is ringing the bell, the neckerchief is gone again.
Revealing mistake: When Sam first sets off in the canoe, it is obviously being towed because it leaves a moderate wake even though he is barely paddling.
Factual error: Suzy's bra is not typical of a young girl in 1965. Training bras in 1965, often manufactured by Teenform or Teencharm, were much more substantial and modest.
Other mistake: When the narrator holds up the wind speed gauge at the water's edge, strong whitecaps should have been visible to match the 22MPH indicated.
Factual error: The VW Bug that the belfry falls on is a later model than 1965 when the movie is set. The larger tail light with yellow on the lens seen on the VW in the movie didn't come along until 1968. The one in the movie may even be of early 70's vintage.