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.
Trivia: In the scene where Doctor Spivey is interviewing McMurphy, this whole scene is improvised. Spivey (Dr. Dean Brooks) is the ACTUAL doctor of the institute in real life, and was simply told to interview Jack Nicholson (McMurphy) as if he was a real patient. Nicholson had to improvise and get from the beginning of the scene to the end.