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.
In the "voluntary/involuntary" scene, McMurphy goes from having five o'clock shadow to being clean-shaven, then back to having stubble.
In the voluntary/committed scene, the writing on the chalkboard behind nurses Ratched and Pilbow is different in three different shots of them.
When the doctors bring back Randle McMurphy from his treatment, they walk in, one doctor puts down his doctor bag, then they lay Randle down. As they walk out, the doctor bag has disappeared and neither one is carrying it out.
During the meeting scene, when the men are tossing the cigarette around, it changes length in between shots.
The Birmingham Church Bombing of 1963 happened in September, but the World Series was played from 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 McMurphy's ECT, when he is escorted back to the ward feigning brain damage, Cheswick and Bromden, both of whom were also given ECT, are already there as though nothing happened to them. It is revealed that all three had recovered, and McMurphy was playing a prank on the others.
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.
McMurphy dials only six numbers when he calls his girlfriend from the Nurse's station.
Differences in clothing between McMurphy and his stunt double when McMurphy drops to the ground on the other side of the fence.
Towards the end of the movie, there are multiple references to it being winter, however, in shots of the outside, there are leaves on the trees, birds outside, and other small details inconsistent with winter in Oregon.
The Monopoly game has plastic houses and hotels. In 1963, they would have been 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.
During their road trip on the bus, they pass a "DUCK XING" road sign. Those did not exist in 1963.
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 McMurphy is "announcing" the baseball game, his reflection in the darkened TV set does not match the audio track.
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.
When it is a first-person view of McMurphy looking through the nurse's room, you can see the reflection of the cameraman in the glass.
Near the end of the party, Rose's microphone pack is visible on her back while she is dancing.
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.
When McMurphy is talking to the doctor in his office, there is the shadow of a mic visible in the left upper corner.
A black microphone standing out against the white ceiling.
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.