During the basketball game, McMurphy runs to the edge of the court, the camera follows and, for a moment, reveals an assortment of film production equipment including lighting stands, C-stands, lighting gels and even a crew member. Only visible in the wide screen version.
After the Chief euthanizes McMurphy with a pillow, his tongue is moving. This could be a reflex action.
The Monopoly game has plastic houses and hotels - in 1963, they would still be made of wood.
After McMurphy hijacks the bus and is driving through town, there are some 1970s automobiles, including a Plymouth Duster and Chevy Nova, and a store with lots of color TVs in the window. The movie is set in 1963, according to the World Series broadcast.
When the doctors bring back Randle McMurphy from his treatment, they walk in, one doctor puts down his doctor bag, they lay Randle down, and as they walk out the doctor bag has disappeared and neither one is carrying it out.
When McMurphy is "announcing" the baseball game, his reflection in the darkened TV set does not match the audio track.
In the "Voluntary/Involuntary" scene, McMurphy goes from having a "five o'clock shadow" to being clean shaved, then back to having stubble.
When fishing, the condition of the water changes as the scene shifts from one person to the other. Sometimes it is wavy with whitecaps and other times it is flat.
When it is a first person view of McMurphy looking through the nurse's room, you can see the reflection of the camera man in the glass.
After the voting for the change of the TV time, McMurphy tells Nurse Ratched at one moment that the voting was ten to nine, while it was actually ten to eight, like he said before.
McMurphy dials only six numbers when he calls his girlfriend from the Nurse's station.
When Cheswick is spilling the drink from the bottle to the jar, it is colorless. But when he is dumping the drink, with the hose, in the mouth of the patients, it is red.
Near the end of the party Rose's microphone pack is visible on her back while she is dancing.
Randall nearly chokes Nurse Ratched to death after she treats Billy so cruelly, yet when he is pulled away there are no marks or bruises at all on her throat.
Differences in clothing between McMurphy and his stunt double when McMurphy drops to the ground on the other side of the fence.
In the card playing scene before the music dispute, McMurphy's hair alternates between unkempt 'bed head' and groomed.
After the Christmas party, and Randall is waiting by the open window for Candy to finish seducing Billy, crickets can be clearly heard outside. Crickets do not chirp in the wet, cold, Oregon winter.
During their road trip on the bus, they pass a "DUCK XING" road sign. Those did not exist in 1963.
In the voluntary/committed scene, the writing on the chalkboard behind nurses Ratched and Pilbow is different in three different shots of them (widescreen version).
After McMurphy, Harding drinks the medicine with his right hand. In the next shot he is finishing to drink with his left hand and holding a newspaper with the right hand.
The Birmingham Church Bombing of 1963 happened in September and Hall, Cagle and Robert Chambliss were arrested September 30 but the World Series was playing October 2nd to 7th. That puts the scene with McMurphy listening to the news and watching the nurses being escorted by Turkle out of sequence.
After Billy is caught with Candy in the room, he runs out stark naked chasing after Nurse Ratched while pulling his pants up, then slipping on the floor landing on his right hip. As his bare butt is exposed, we can see a red mark on his right hip from previous takes before he falls on the floor.
During the meeting scene, when the men are tossing the cigarette around, it changes length in between shots.
As Mac is being interviewed and they are looking at the fish, Dr. Spivey is holding a folder on one shot and a pencil in the next.
During the fishing trip, when McMurphy comes out of the cabin we see Candy behind him wearing only panties and holding her shirt to her chest. After a two-second cutaway, we return to see Candy wearing the shirt. She could not have gotten dressed that quickly.
McMurphy's hat when strangling Nurse Ratched.
As McMurphy dances with one of the patients while trying to convince more people to vote in favor of watching the World Series, the shadow of a camera is plainly visible on their backs.
Just before McMurphy climbs over the fence to hide in the bus, what looks to be a Ford Pinto can be seen in the hospital parking lot.
When Nurse Ratched asks for her cap, she reaches for it with her left hand. Then there is a cut to close-up, and she grasps the cap with her right hand.
a black mike standing out against the white ceiling.
As the chief and the boys are being escorted through the doorway on the way to their ECT, the male orderly with Chief is not the same one as the one on the other side of the doorway. The second one is much taller, nearly equal in height to the Chief.
Towards the end of the movie, there are multiple references to it being winter, the most obvious are: the television announcer mentions the "upcoming holiday season", and the morning after the party, a sign saying the date is December 11th can be briefly seen. However, in shots of the outside, there are leaves on the trees, birds outside, and other small details inconsistent with winter in Oregon.