Deliberate mistake: When King Willie is confronted by the Predator in the alley the Predator approaches him cloaked stepping through the puddle of water. You can see his footsteps causing the water to splash up. However, you can see the water splash up and go through his foot. The Predator is only cloaked, not intangible so the water should have splashed upon his foot, not go through. Water against his foot would be visible too, even though he is cloaked. (00:47:45)
Deliberate mistake: Morse code takes a lot longer to type out than the captain of the Dallas seems to need. Likely "compressed" because otherwise the process would be quite dull, but still.
Deliberate mistake: When Bastian comes running up the stairs by the harbour, you can see a black couple on the right. The woman is wearing a grey coat with a black vest and a white shirt. The man is wearing a black coat, black shoes and white socks. When it cuts to Bastian running across the street, you can see the exact same couple wearing the exact same clothing crossing the street, but this time they are not walking together. The extras were used twice. (00:11:10)
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: While Cohagen is on Mars, he calls Richter and chides him for trying to kill Quade. Richter is in a car on Earth, yet they're having a real-time conversation, which is impossible because of the distance. [While this is true, a movie with 4 to 21 minute pauses between lines would take rather a long time].
Suggested correction: This movie takes place an undisclosed amount of time into the future. Absolutely no telling what kind of technological advances could have taken place, in a science fiction movie at that.