Continuity mistake: When Suzie is looking at herself in the mirror having a cigarette, she has make-up on and some of it is wrongly smeared around her eye. When she is sitting on top of Joe a few moments later, the smeared make-up around her eye is gone and she is just wearing make-up. (00:22:40 - 00:23:15)
Continuity mistake: When Sara and Cid are practicing maths on the multiplication grid, Sara asks what is 8 X 7 and Cid puts a 56 on the board. The 56 is located one next to the end grid in the first shot, but by the next shot, the 56 is suddenly located at the corner of the grid. (01:12:10)
Continuity mistake: In the scene where Old Joe (Bruce Willis) is forced to the time machine and then fights back, the amount of Gat Men changes. 2 are seen bringing him in, another collecting the gold and the brown jacket, and a fourth opening the time machine. However, when he fights back there are only 3 bodies. It is plausible but unlikely that the fourth man would have left and gotten far enough away not to hear the fight.
Revealing mistake: Considerable efforts were made for this film to have Joseph Gordon-Levitt resemble Bruce Willis (colored contacts, nose prosthetics) since they are playing the same character at different ages. Yet the two actors' ears remain distinctly different. Bruce Willis's ears are much larger than Joseph Gordon-Levitt's, and Joseph Gordon-Levitt has connected ear lobes while Bruce Willis does not.
Continuity mistake: In the diner shootout scene, Joseph Gordon shoots at Bruce Willis, missing and smashing the window that's right next to the blue lighted sign on the wall. After more loopers arrive to kill Bruce Willis, the window is intact once more and Bruce shoots it out again so he can jump out of it. (00:51:20)
Continuity mistake: While hiding from the Gat man in the tunnel made by Cid's grandfather, the stick used to hold open the trap door switches from the knot being at the side, to the knot being at the front.
Continuity mistake: When Kid Blue is holding his Gat gun on Joe he pulls back the hammer. In the second shot of Kid Blue, the hammer is back to its original unpulled position. Then, just before Abe swings open the door the hammer returns back to its correct pulled back position. Despite not being able to see the hammer from the angle on the gun we are given, the location of the hammer is given away by Kid Blues' thumb placement. (00:15:42)
Continuity mistake: When Joe is escaping the diner on Seth's bike, Kid Blue is shown running up to him. At this point, Kid Blue has a blue shirt on, then as he gets blasted back by the bike's engine and falls backwards, he suddenly has a purple shirt on. A few shots later, he is back to having the blue shirt on. (00:49:50)