Continuity: In the scene where Caden is talking to Hazel directly after having talked to the doctor after his seizure, there is a dog in a box behind Hazel in her box office. Upon cutting to Caden, and then cutting back, the dog is gone. This is the remnants of the character "Squishy", from the original draft of the script. The almost-dead dog was found by Hazel after driving home from the premiere. She was saddened by Caden denying her, and she finds the dog, run over and bloody on the side of the road. She decides to keep it. This is the only scene where he is present, and his presence is not explained.
Incorrectly regarded as goofs: Announcer on the radio at the very beginning says it's 22 September. The newspaper is dated in October, it's Christmas when the sinks smashes his forehead, New Year's on the ride home and March in the ophthalmologist's office. Kaufman afforded his film a dreamlike quality by playing with the representation of time throughout.
Anachronisms: Near the beginning when Olive asks if she can watch television before school, Caden picks up the remote and turns on a television with rotary knobs that would not have remote control functionality.
Factual errors: In both instances where Caden is posting presents to Germany for Olive, the packages would never arrive - for one because there is no 'Kampfstrasse' in Berlin (though this would literally mean 'Fight Street', most likely a poetic choice made by the filmmakers), but also because the postal code he has written, D-1805, doesn't exist anywhere in Germany. German postal codes are all 5 digits in length, and the ones allocated to Berlin range from 10001 to 14199.