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: 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.