Continuity mistake: When Joseph Gordon-Levitt's character is shot by Emily Blunt with rock salt he is hit in the right shoulder. In the next scene he is having his left shoulder treated.
Continuity mistake: When the waitress delivers the identical breakfast plates to the two Joes, she places them in exactly the same way with respect to the men - eggs toward them, steak facing away. In the longer shot, old Joe's is different, although neither has touched the plate.
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 shooting the window out that's right next to the blue lighted sign on the wall. After more loopers arrive to kill Bruce Willis, Bruce shoots the same window that was already shot out in the next scene in order to jump out of it.
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.