When Joy steals the truck a black guy is unloading it. When Joy drives away it's obvious there's no-one in the back of the truck.
When Joy and Earl open the back of the truck later, they find there's a white guy in the back of the truck. So this had to be a black guy, or there were two of them unloading.
When Joy steals a truck, there's a guy locked in the back. When any interior shots of him are shown, the back of the truck is well lit. However, the back of the truck has no light, so the interior shots should be in the dark.
When the stolen truck runs dry, Earl walks away to get gas. As the camera fades from a close shot to a longer one, the back of Earl's head in the new shot briefly looks like a large poop stain in the previous one.