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 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.
Revealing mistake: In the iconic Big Kahuna burger scene, Brett gets shot in the shoulder, a normally highly debilitating injury. However, in the frame before he is killed, he throws back his arms with the ordinary range of flexibility, and it becomes obvious that the actor's mobility is not really impaired, and he was only faking an injury.