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."
Revealing mistake: When Butch is riding in the taxi and smoking, the cigarette smoke is lazily drifting out of the window of the vehicle. If they were traveling at any real speed (i.e. not on a set with a movie screen behind it) their smoke would be ripped out of the taxi by the wind. Also, when he tosses out some of his stuff you can hear it hit the wooden floor of the stage.
Revealing mistake: When The Wolf sprays down Jules and Vincent with the hose, they are supposed to be naked. When Jules hands Jimmie his clothes, you can see Jules is wearing tan colored underwear at the very very bottom of the screen. You can only see a little bit of it, and it is a very quick shot.