Continuity mistake: In the scene at the apartment with Brett, Vincent closes the briefcase and leaves it on the kitchen counter. He continues to smoke until it's almost time to shoot Brett. They shoot Brett and the scene fades out to be continued later. Later we see that after shooting Brett, Vincent is focused on Marvin still in the corner having a panic attack. Jules tells Marvin to knock it off and the man comes out of the bathroom. Jules and Vincent shoot him, then Vincent chides Marvin for not telling them there was someone in the bathroom. Vincent only has his gun in his hand, no briefcase. Jules and Vincent argue about 'the miracle' and they leave. Marvin never leaves the corner until Jules tells him 'let's go'. Nobody has the briefcase.
Continuity mistake: At the beginning, when Brett is initially shot by both Jules and Vincent after Jules' Ezekiel speech, we see Jules' gun becomes empty on firing the last shot. However, on returning to the same scene at the end, when Jules and Vincent shoot Brett the gun does not empty; in fact Jules is able to repeatedly shoot the guy who comes out of the bathroom afterwards and the gun still isn't empty.
Continuity mistake: When Mia and Vincent arrive at Jackrabbit Slims, they stand in front of a desk and confirm their booking. Throughout her exchange with the 'greeter', Mia's fringe is swept to the side, looks a bit greasy, and is separated. When they go to walk to their table in the next shot, it's suddenly perfect and covering her whole forehead.
Revealing mistake: When Lance opens his front door to see Vincent driving across his lawn, with an over-dosed Mia inside the car, the car passes to the left out of view, then we hear the sound of it crashing to a stop against the house. Lance goes outside to see just that as Vincent carries Mia out of the car. If you look at the wheel marks on the lawn, you can see that the car has in fact driven past the house, and out of sight, and that the car in which Vincent and Mia came from was already obviously there in "crashed mode."
Factual error: When Vincent shoots Marvin the bullet goes through Marvin's head and exits out the back of his skull with explosive force - enough to splatter "pieces of brain" all over the back of the car. The rear window of the car should have shattered from the bullet, it doesn't even get chipped.
Continuity mistake: When Vincent takes Mia home after giving her the shot, Vincent and Mia talk, then Mia finally tells Vincent the Fox Force Five joke. Before and while she tells the joke, in every shot behind her, her arms are down by her sides, yet in every front view of her, her arms are folded, or one arm is up by her mouth.