Susan Henning took on the role as Hayley Mills' body double for several of the twin shots in the film. As part of her contract, she signed away her rights to be credited. At the wrap party, Walt Disney presented her with a small statue of Donald Duck, called The Duckster, in recognition of the "best unseen performance on film." Only 2 other Ducksters have ever been awarded. To her credit, Ms. Henning did later star with Elvis Presley.
Daphnis and Chloe are the title characters in the only known work of the 2nd century AD Greek novelist and romancer Longus.
For the scene where Maureen O'Hara and Charles Ruggles are speaking in the bedroom, Ruggles didn't have any place to put the ashes of his cigarette so, on his own, he decided to put them in his hand.
Pelléas and Mélisande are the title characters in a Symbolist play (by Maurice Maeterlinck and was first performed in 1893.
The "isolation" cabin that Sharon and Susan stay in at camp is named "Serendipity".
The screenplay originally called for only a few trick photography shots of Hayley Mills in scenes with herself; the bulk of the film was to be shot using a body double. When Walt Disney saw how seamless the processed shots were, he ordered the script reconfigured to include more of the special effect.
The title song was performed by Tommy Sands and Annette Funicello. They were on the lot shooting Babes in Toyland.
This was Brian Keith's first comedy participation.