Goofs

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.

The blood the hitchhiker smears on the van is gone in the long shot a few seconds later.

After the hitchhiker cuts his own hand there is a cut to him inviting them to dinner and his hand is clearly uninjured.

When Sally jumps out of the window for the second time, there are clearly visible glass shards all around her. When camera changes to another angle the glass shards disappear, never to be seen again.

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.

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.

None of the characters seem to realize that the Sawyers were Sally and Franklin's next-door-neighbors. At the very least the Cook should have recognized them when they told him where they were headed (it's only been roughly ten years since they moved away, the Cook was a grown adult at that point) but it never gets brought up or even acknowledged at any point in the film.

CORRECTION: It's possible that the Sawyers did not inhabit the home simultaneously as Sally and Franklin's next-door grandparents. It's also not known know for sure if the Sawyers are the true owners of the property since they could have likely slaughtered the previous occupants.