The blood the hitchhiker smears on the van is gone in the long shot a few seconds later.
When Leatherface chases Sally into the house the first time and she escapes through an upstairs window, he corners her on the stairs and she leaps out a window off the hallway on the second floor. However, when Leatherface appears in the empty window frame after she jumps, he's standing in an attic window with a gable.
After the hitchhiker cuts his own hand there is a cut to him inviting them to dinner and his hand is clearly uninjured.
When the group of teenagers pick up the hitchhiker, he goes crazy and cuts Franklin's arm, so they throw him out. As they throw him out, the van-door windows are shut. In the following inside shots, they're open.
When they get to the gas station early in the film, it constantly changes from overcast skies to the sun brightly shining with each different camera angle.
After the hitch hiker cuts his own hand, he laughs and waves the knife. Watch carefully and you can see the hose behind the blade the blood came through.
When the Hitchhiker has been thrown out of the van and proceeds to chase after, and smear blood on the side of the van, at one point the camera cuts to the inside of the van to show the hitchhiker blowing raspberries through the window at the teenagers. During these perspectives, a clapper board, and a crew member wearing a red t-shirt, can be seen reflected in the window.