Deliberate mistake: After the first scene where Hannah's ex-husband walks in without knocking and picks up the kids, Hannah is looking at some family pictures including her, her ex-husband, and the kids (supposedly taken when they were still married). But in the pictures they're all wearing the exact same clothes that they were in the previous scene. Could be a coincidence but that seems doubtful.
Deliberate mistake: The elevator operated by gremlins starts ascending at around 5 stories per second, based on the floor readings. Yet when the elevator suddenly stops dead, Kate does not fly into the air caused by the huge speed change. (00:50:05)
Deliberate mistake: In the hotel bar at night, when Edward is playing the piano and Vivian goes to find him, after Edward has dismissed everyone else from the room Vivian sits on top of the piano and puts her feet on the keyboard. Her feet go onto the keys in the middle of the keyboard but the sound that comes out is from the bottom and the top notes of the piano.
Deliberate mistake: The same shot of the railway station concourse is used when Graham and George are heading to work, and also when George commits suicide. The giveaway is that several people are seen in the same position. (00:05:00 - 01:20:30)
Deliberate mistake: During the train sequence at the end of the film in 1885, it is blatantly obvious that the train is not going the speed that the DeLorean's speedometer is showing. In some shots, the train looks as though it is going little more than 20mph. (01:38:55)
Deliberate mistake: When Casey is hiding behind the boxes waiting to knock out a Foot soldier and steal his outfit, you can see he is still wearing his denim vest. After he knocks out the Foot soldier, he is able to put on his entire outfit in roughly three seconds. (01:10:15)
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)