Other mistake: When Prot visits the doctor's house, he thanks the wife and the camera is zoomed into his sunglasses. The reflection of his glasses shows the doctor looking at him, but the doctor is standing behind him.
Continuity mistake: In one of the first family scenes, the youngest daughter shows she has lost a tooth. The next shot, the tooth is back in her mouth.
Continuity mistake: The taxi ride to the planetarium begins in a Ford Crown Victoria but ends in a Chevy Caprice.
Continuity mistake: When Jeff Bridges is in front of Robert Porter's house, with the sheriff, you can see that the shadows of the background mountains are different to the people's shadows.
Continuity mistake: The bowl of green jello that Doris throws at the warden lands the right way up, but in the next shot it is upside down.
Continuity mistake: When Prot takes his sunglasses off for his "return" (because he would supposedly no longer need them due to the lower light levels on K-Pax), he places them on the nightstand upside-down, with the earpiece flush to the table. Two subsequent shots (including an elaborate effects shot) show the sunglasses right-side-up.
Continuity mistake: When the doctor shuts all the blinds in his office, and then interviews Prot, his left hand collar button is undone/done up in different shots.
Other mistake: When we first see Jeff Bridge's character on the elevated subway, the car is jostling around in obvious motion. However, the scenary visible through the rear door window of the train is clarly not moving in response to the train. The city background is not moving farther away. The scene quickly changes angle and the background now begins to "move" in relation to the progression of the train car.
Plot hole: When the "Blue Bird" patient finally sees the Blue Jay, he starts excitedly shouting "Blue Bird! Blue Bird!" This uprising causes a riot in the institution, and in the immediate exterior shot of the building, everyone from all four floors already know and are staring out the window at the bird. Within the time-frame of the scene only the people on the floor which saw the bird would actually know about this. Psych ward floors are kept separated as it is even noted by an attendant when he said, "Do you want to go to the fourth floor?" which indicates that the floors are kept separate. This was deliberately done for dramatic purposes.
Factual error: At the end of the movie, the son arrives from Dartmouth. But he does so in Grand Central Station. To come from New Hampshire he would have to arrive at Penn Station.
Visible crew/equipment: When Jeff Bridges takes Prot to meet his family the crew is visible in the reflection in his glasses as he shakes Jeff's wife's hand.
Continuity mistake: When the woman who never leaves her room is seated at the table with some other patients, she's talking to the doctor and closes her hand fan. The very next scene shows the fan open again.