Asking for Prendergast's signature the delivery man says, "put your John Doe right here". A signature is often called a John Hancock, but never a John Doe, which is law-enforcement lingo for a male suspect whose name is not known.
When the gang member Angie (Karina Arroyave) is doing her best to describe F-Fens (Michael Douglas), she points to Predergrast (Robert Duvall) and says, "he looked like him only taller". Michael Douglas and Robert Duvall are the same height.
In the burger scene, after Bill shoots, an old man tries to escape. At the beginning of the next scene, you can see the same actor, dressed differently, in another restaurant. Both events are supposedly occurring simultaneously.
Just before the park scene when the people are getting on the crowded bus, you can see a black woman with shorts and a blue shirt all the way to the right get on the bus; you can see her in front of Bill as he tries to get on the bus, that is the top of her head as he gets bumped in the crowded line. However, in the next scene she is dancing in the park with a headphone radio.
When Bill's wife leads the lady police officer out of the house, she turns, closes, and maybe locks the gate to her front yard. When Bill finally arrives, when his wife leaves the house from the back door, he runs through an open gate up to the front door.
When shooting the phone booth, the man watching as happens in front of his eyes is changing hands from inside his pockets (in the long shot) to waving them around (the closer shot). There is no time between these shots to move his hands.
In the Whammy Burger scene, after D-FENS holds the place up, he asks for his breakfast, and then he changes his mind and asks for lunch. After he goes around the room talking to customers, Rick the manager has his order on the counter; a Whammy Burger, a shake, and fries. D-FENS grabs the burger box and fries, but when he opens the burger box, the fries disappear.
When D'Fens is at the Korean's store and is first going to leave without his Coke, his briefcase is in his left hand. He turns back to the counter to argue with the store clerk. When D'Fens decides to stay, he slams his briefcase on the counter, only now the briefcase is in his right hand.
In the opening scene, when the camera first pans onto the car with the Garfield doll, two big flat light-diffusers can be seen in the half-open rear window. If you look even closer, the boom mic and some people are visible standing behind the diffusers.
In the store when Bill calls his wife and makes the Apollo 13 reference, the spacecraft was actually prior to the point of no return when the service module exploded. The reason why the spacecraft couldn't return early was because was because the service module near the main engine was damaged and they did not want to risk lighting the engine up for fear that the ship may explode. A free-return trajectory was advised and they returned safely.
Earlier in the film Prendergast's wife tells him to pick up chicken for dinner, however later in the movie he tells her his dinner better be ready when he gets home even though he never went and got the chicken to make for dinner.
When D-FENS checks his watch at the Whammy Burger, it is 10:33, three minutes past when service cuts over from breakfast to lunch. However, as he looks/walks around, the restaurant is full of customers midway through their lunch food, despite it just becoming available three minutes ago.
In the opening scene some people think that Bill is trying to roll his window down when the crank mechanism stops working, making it impossible for the police officer to reach through the window to steer the car off the road when it's being pushed. But Bill is not rolling the window down - he's trying to roll it UP to block out the sound of the outside world. In later shots you can see that the window is about half open, making it possible for the police officer to reach through to steer the car.
The crack on Bill's right lens in the second half of the movie appears to vanish at some points but this is just a trick of the light - when Bill faces directly into the camera, because the frame is undamaged and holding the cracked lens firmly in place, the crack is very difficult to see but still just about visible if you look closely.
When he confronts the two men in his store while D-Fens is looking at boots, Nick ("Surplus Store Owner") pulls what appears to be a nickel-plated revolver from a holster on his right hip. A short time later, in his back room, Nick again draws a handgun from his right hip to subdue D-Fens, but this time it's a semi-automatic pistol with a dark (perhaps blued) finish. It's unlikely that Nick would be carrying two different handguns on the same side.