Continuity mistake: When Joseph Gordon-Levitt 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.
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: When Joe exchanges the silver bars, his friend is shown entering in the background. The man places his hand in the right hand corner of the basket as he finishes placing the gun. A few seconds later, as Joe retracts his gun from the basket, the man's gun is now placed at the left hand corner.
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.
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 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.
Continuity mistake: After Joe puts his gun in the basket at the start, he walks over to a slot to put the silver bars in. A CCTV camera is visible duct taped to the wall with a wire attached to the camera. The wire next to the camera goes on for a few inches before there is a bend in the wire going upwards. After Joe says "Two, Jedd" the shot cuts to a close-up of the CCTV camera, and the bit of wire before the bend is shorter than it was before.
Continuity mistake: In the "loop closing" montage, after Joe is shown at the cafe speaking in French to the waitress, he is shown looking at his stopwatch and the time reads 2:29. The following shot shows a tied up man appearing from the future on the white sheet. Joe then shoots the man, but it is noticeable that the angle of the gun and the position of the wound after he is hit do not match up. The gun itself is aimed at the man's head, possibly higher, but the wound itself is shown in his chest.
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.
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.