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 "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.