Revealing mistake: When King Arthur is fighting the Black Knight, when he is about to chop off his last leg we can see the Black Knight jump slightly, after he has done this we can see his leg fall before King Arthur has touched it with his sword. (00:15:25)
Revealing mistake: You can see the threads moving the submarine underwater. (00:04:55)
Revealing mistake: You can see the wires on the handgun pointed at the two extraterrestrial children when the bad guys are staring in shock at the "floating gun."
Revealing mistake: During the chase sequence towards the end of the film, Roger shoots five bullets into the windscreen of the blue car. If you look carefully, you can see there is no driver in the front seat.
Revealing mistake: When the protagonist jumps two of the evil cowboys that are after his jewels, the stuntman on the right tosses aside the rifle in a quick deliberate move pretending to lose it. (00:10:25)
Revealing mistake: When Hettie drives her motorbike up to the café for the first time, you can see the horse and cart in the background zoom across the frame - because of the film being sped up at that moment.