Continuity mistake: In the scene where character William Cage arrives by helicopter, as he exits, a soldier approaches to take out bags/suitcases. In the following shot (from a distance) Cage walks away from the helicopter, but he's still very near the helicopter. The soldier unpacking the helicopter is walking towards the helicopter however, again picking up the bags. (00:03:20)
Continuity mistake: When Cage lands on the beach for the first time, Kimmel gets crushed by a downed transport. The force of the crash knocks Cage to the ground. When Cage starts standing up, a transport can be seen in the upper right corner of the screen. When you first see the transport, it is closing its bay doors and banking left (left on screen, but moving right to the viewer). A closeup of Cage standing up then blocks the view of the transport. When the camera pans back from Cage, you can see the transport again. Except it's doing the exact same thing as before in the previous shot of it, closing the bay doors and beginning to bank to the left. (00:19:55)
Continuity mistake: When Cage does 50 push ups he looks back and the two trucks are coming straight. In the second shot where he looks back at the trucks again the second truck is just turning straight. (00:36:40)
Continuity mistake: In the first encounter with Brigham, Cage has one button on his jacket undone during one shot, and in the next shot it is buttoned up.
Continuity mistake: When Cage and Rita leave the underground garage at Whitehall, the front wheels and bumper get damaged by the ramp. In the next scene they are undamaged.
Continuity mistake: When Cage wakes up in hospital, there are two bags of blood, both nearly full, but when we look away, then back again, the blood bags are nearly empty.
Continuity mistake: In the office, there are two glass display cabinets, and Cage stands in front of the left one. In one close up shot, the contents of the right one can be seen behind him.
Continuity mistake: The dirt on the window screen changes throughout the caravan park and driving scene after, from very dirty to virtually clean.
Continuity mistake: Rita's scar above her right eye is sometimes very prominent in some scenes, then very faint in others, and in some scenes it's not there at all.
Chosen answer: First it should be noted that this film is based on the Japanese novel "All You Need is Kill" by Hiroshi Sakurazaka, which is where the term "mimics" comes from and could be something lost in translation (they adapted to alien planet environments by mimicking the biology of the planet's life, not necessarily making themselves look like a certain species). However, in the film itself it, was said the aliens "mimic our every action", militarily speaking. This is of course because the aliens were resetting the timeline when an alpha died and knew what the military would do, but to the military (and news reporters) it looked like they were just copying our fighting style.
Bishop73