Continuity mistake: When Mc Murphy is strangling the nurse at the end, from above his hat is lying on the floor, but when you see the shot from her angle, he is wearing it again.
Continuity mistake: You see an opening scene where McMurphy is arriving by automobile. When he gets out of the car it is a cloudy dark day. A few seconds later when he is being brought into the hospital with the guards we see them entering the front door together from down the hallway. Now you can see that it is a bright sunny day outside.
Continuity mistake: In the scene where McMurphy gets upset while playing blackjack, he gets up and walks towards the nurses' quarters to turn down the volume on the record player. Martini and Bibbit are still sitting at the card table while McMurphy is walking towards the nurses' quarters but when he gets inside, Martini is already leaning against the counter of the nurses' quarters with Bibbit close behind him.
Continuity mistake: In the scene where the patients are arguing over cigarettes and tossing one around from patient to patient, the cigarette varies in length from long to short and back again as you see different shots of patients getting the cigarette.
Continuity mistake: In the scene where the boys are playing blackjack, watch McMurphy's cigarette as he talks to Martini about the rules of the game. It changes length throughout the scene.
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.
Continuity mistake: When the guys go fishing on the boat and one of them catches a fish, they panic, and the boat starts to go in circles. In the close up shots and the wide shots in that scene the sea is very different from shot to shot.
Continuity mistake: In the "therapy" session where McMurphy discovers that most of the inmates are here voluntarily he first looks as if he hasn't shaved for days, but in the second half of the scene he is clean-shaven. In the same session the writing on the board behind Nurse Ratchett changes: It says "No smoking in pool area", but when she says "You're making some very challenging statements" the text disappears for one shot.
Revealing mistake: If you look closely at the scene where McMurphy climbs the fence and falls from the tree, Nicholson's stuntman looks nothing like him.