Revealing mistake: Obviously not Jane Seymour sitting on the bed topless as her back suddenly looks tanned.
Deliberate mistake: After Ryan and Michelle finish changing the flat tire and are surprised by Leatherface, he chases Ryan around the back of the car and smashes the windshield. At the moment the camera is in the backseat shooting out, we see the chainsaw with the chain popped off and hanging. My best guess is that it was done intentionally (for safety reasons) because the saw was close to Ryan. The crew must've been too lazy to completely remove the chain and just popped it off the track instead. (00:25:25)
Continuity mistake: A helicopter drops two harnesses down to Chuck Norris (Major McCoy) who is awaiting with an unconscious bad guy, and tethers the villain to the ropes dropped by the hovering copter. The black harness ropes are shown dangling from the chopper throughout. When McCoy radios to "pull us up" with the villain in view, the once visible ropes are nowhere to be seen. (01:41:40 - 01:42:05)
Character mistake: G. Peppard holds up a Walther P-38 pistol and calls it a Walther PPK.
Continuity mistake: When Gatou Vardebedian is holding Gus Cardinale at gunpoint, the gun keeps changing position between shots (under his throat, by his nose...) and she is holding it straight or turned sideways. (00:13:00)
Revealing mistake: At the start of the film, where West and Cain are re-animating dead soldiers, one of them tries to kill West. So, he shoots him in the head. It's obvious that West's gun doesn't fire, and the soldier just pretends like he's been shot.