When playing volleyball on the beach and Juliette goes to speak to Ken, the shot cuts back to the volleyball match and Juliette can clearly be seen playing. In the next shot, she is walking back towards the game after having spoken to Ken.
At the beginning of the film, the teenagers are playing volleyball and Sarah has shorts on; then she goes from having a bikini bottom and shorts back on her. Then, she has the shorts in her hands and picks them up. Later when Jackie and Diane are talking - in the background, Sarah has her shorts back on.
When Duncan and Frank are murdered, their bodies are left on the floor of the foyer. In subsequent shots, the bodies have been removed and don't reappear until Jackie, Maria, and Diane head down to the basement.