When Sofia is holding the present for David at the party. She gives it to David, then she is seen holding it in the next shot and then gives it to David again.
During the party at David's house, Julie's hair is parted to the left, and later, it is parted to the right, while still at the party.
When Sofia is looking at the drawing that David made for her, we see her right hand move across the picture as she starts speaking. In the following shot her right hand is holding the picture.
When David goes to see Sofia for the first time after the accident, her ponytail is hanging in front of her shoulder. In the very next shot, it's hanging down her back.
During the party, when David and Julie are upstairs in his bedroom kissing, Julie's head is tilted to her left when shot from in front of her, but tilted to her right when she is shot from behind.
When Julia and David both drive off the bridge they both should have gone through the windscreen, over the car bonnet and face planted the wall as neither were wearing seat belts at the time of impact.
In the closing credits, there is a credit for The Who's performance of "Baba O'Riley" from the documentary The Kids Are Alright (1979). Actually, the short clip of The Who (seen during the montage in the final scene of the film) is footage of their performance of "Won't Get Fooled Again" from the same film.
In the opening scene, David Aames awakens to the sound of his alarm clock, and quickly turns it off. But in reality that alarm clock was a JVC FS-SD1000 and the button David touched was the volume down button which would not have stopped the alarm nor made an indicator sound upon being touched.
When David and Brian are in the car in the beginning you can clearly see that they are about one or two feet higher compared to the other cars, even though they are in the relatively low Mustang, revealing that the car is probably on a trailer rather than on the road.
Where David points to the mole on Sofia's chest, she chuckles, yet her facial expression remains completely unchanged.
When David and Brian are almost hit by the Mack truck, we can clearly see the camera and camera operator (wearing a hat) in David's side window.
In the Times Square sequence, in the far background you can see pedestrians standing watching the filming of the movie.
When David awakes after sleeping in the street, the camera and equipment are reflected in the street puddles.
When Brian tells David that the "new guy is shit" and then he says to David, "I love you, period," you can hear the crew laugh at this in the background.
During the party, when Steven Spielberg makes a cameo appearance and says happy birthday to David, a cameraman is seen filming on the left.
As David returns to his car the morning after his night with Sofia, Julie approaches, having waited for him in her car down the street. How could Julie have known where to find him? Julie didn't know Sofia. Julie was still at the party when David and Sofia left. And from the setting and street sign it's clear that Sofia lives near the Manhattan Bridge (Water St.) in Brooklyn, an entirely different part of New York City than David (who lives in The Dakota across from Central Park), so Julie couldn't possibly have found him by just driving around searching. Brian could have told Julie where Sofia lives during the party, but he just met Sofia earlier that day and it would have been an odd thing to do, if he even knew Sofia's address yet himself.
Ventura says that David was completely withdrawn for "months" after wresting control of the company back from the board of directors. However, a plaque seen near the end of the movie gives David's birthdate as "9-22-68," so his 33rd birthday, and the party, would have been on September 22, 2001, and the same source shows that an extremely significant event involving David happened on December 26 of the same year, not leaving enough time inbetween for him to have been a hermit for "months."
In the scene where David is taking his "mug" shots, it shows his height as just shy of 6', but he is 5'7".