Continuity mistake: Lisa drives Sam to the hospital and is met by Dr. Marsh. Dr. Marsh puts on a protective eye mask, before administering a syringe of luminous substance into Sam's arm to stop him projecting fire, but the next scene shows him not wearing the mask then it's back when disposing of the syringe.
Continuity mistake: During the final battle between Boone and Decker, at one point, they are wrestling on a wooden bridge, and the board beneath Boone's head breaks in half and falls into the cavern below. (It is even highlighted in a very dramatic shot looking downwards over the whole cavern as it falls) Yet, a few shots later, the board has suddenly re-appeared beneath his head, unbroken.
Revealing mistake: When young Norman Bates is spying through the peephole on his mother, who has brought home a man to the motel, we see him in the shower. Although the curtain is translucent, we can see he is wearing a black speedo.
Continuity mistake: When Dr. Jekyll sits down to try his formula for the first time, he removes his ring from the ring finger of his right hand and puts it on the table next to him. After drinking the formula, as his transformation into Hyde starts, his ring is back on his ring finger.
Other mistake: The hole that the guy got his arm torn off in was too small to fit into, despite the monster being on the other side. A few moments later, he's pulled in. If the opening was too small, the body would be crushed and blood would spray everywhere, but you see none.
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: When the zombie Barry falls on Jordan, he's played by an obvious stunt double who isn't wearing any zombie make-up.
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.