Revealing mistake: When Barnes and Marcus Wright are helping John Connor back to the chopper and when Barnes gets in the helicopter, he's revealed to be a stand in. It's in plain sight, at the right side of the screen. (01:41:20)
Revealing mistake: After Marcus rips out his chip implant, he picks up a steel stool and throws it through the screen. The stool stops in midair as soon as it breaks the screen, which is a mistake in itself. The stool does so because there are wires connected to the stool, at least one of which is visible: a microfilament wire from which the stool is suspended. It is attached to the the stool at the pole between the upper and lower plates. The displays on the other computer monitors in the background appear to have been meant to help disguise this, but one line in particular moves in perfect sync with the stool. Also, the wire snaps taut, then wiggles with slack, whereas the lines on the monitors remain completely straight at all times. A previous correction claims that no wire is visible, but that is completely erroneous. (01:30:00)
Revealing mistake: When the tow truck is dragging the bike, as it goes onto the bridge the drag line is visible in the mud from previous takes.
Revealing mistake: In the final scene when Marcus get John in the helicopter he is wearing a glove with markers where his metal hand should be.
Chosen answer: It was a strike by Skynet on the attacked facility to try and deny the resistance information and anything useful.
GalahadFairlight