Other mistake: In the scene where the Chief smothers McMurphy, when the Chief pulls the pillow away from McMurphy's face, you can see McMurphy's tongue moving up and down even though he is supposed to be dead. (02:03:45)
Other mistake: The movie is a time piece set in the year 1963. After McMurphy jumps the fence, there is an orange/red Ford Pinto in the scene. Ford Pinto's weren't introduced until 1970. (00:51:27)
Answer: The girls who sneaked into the ward had brought alcohol and everyone was partying before McMurphy and Chief were to escape. Everyone got drunk while waiting for Candy and Billy to finish, eventually passing out. Candy and Billy probably just assumed McMurphy and Chief had already left or they'd fallen asleep.
raywest