Continuity mistake: During the final shootout, when Ritchie comes down the stairs, there is one too few bullets fired for the hits on the walls.
Continuity mistake: Sailor Ripley and Bobby Peru are robbing the Feed Store in Texas. Bobby says "Say cheese" shooting one of the two men. We can see that his right hand is blown off. Cut to a shot of the two men searching for the hand and the victim has a bloody left stump. Then the store dog runs out with a right hand in its mouth.
Factual error: This film is set in Casper Wyoming, yet at the end, as they are pulling away, a visible sign for Interstate 80 can be seen. Interstate 80 doesn't go through Casper.
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: 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.
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.
Other mistake: Tough beat cop Farley Wood in his spare time built a special electronic machine to feed his cat. He needs to take off for Miami at 3 PM. When his cat is eating before his departure though, the clock on the machine says it's "12 AM", which would be midnight, not noon.
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)
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)
Character mistake: G. Peppard holds up a Walther P-38 pistol and calls it a Walther PPK.