Goofs

Joe is right-handed, whereas Old Joe is left-handed. Not only that, all of Old Joe's mannerisms - the smirk to one side, the head tilt, and so on - are mirrored.

When the mob is preparing to send back Old Joe, there are four gangsters: two restraining him, one attaching the gold bars, and another preparing the time machine. In the fight that follows, there are only three of them.

When Joe is talking to Cid in the kitchen at night, Cid has a noticeable gap in one of his front teeth. Later, when Cid is talking to his mother Sara, he still has a very wobbly baby tooth where the gap was previously.

When the truck is upended and overturns, it is clear that there is no one in the cab. When it then comes to rest upside down on the road, the close-up reveals Sara and Cid being held inside by their seat belts.

As the Gat men are mobilizing at Abe's and filing out, on the video screen to the right, the same figures pass by over and over.

Whenever anyone is shot their body flies back several feet. This is only possible if they'd been hit by cannonballs fired from cannons as the recoil of the gun is equal to the force of the impact.

Joseph Gordon-Levitt went through subtle prosthetic makeup around his nose, lips, and brow to look more like Bruce Willis, but the makeup artists did not synchronize the actors' ears. Willis has lobed ears while Gordon-Levitt has attached ears. Human ears lengthen over time by about half a millimeter per year. Over 30 years, this is 15 mm, or a bit over a half an inch. Most of that goes into the lobes. It's not exact, but it's close enough for movies.

When retired loopers are sent to their deaths in the past at the hands of their former selves, their faces are covered but they are identified by having gold bars strapped to them in place of the silver that is strapped to other victims. It would make more sense to either not identify them at all or to send them to a different killer, thereby avoiding any temptation for the killers to have second thoughts about killing their older selves.

The loopers don't know they're killing their future selves until they open the payment bag and see the gold. Until they "get a golden payday," they think it's just another person from the future.

Young Joe disposes of bodies by throwing his victims down a shaft into the fire. The body seems to slide horizontally towards the fire instead of vertically falling into it.

When Jesse comes to Sarah's farm to investigate, Cid (Sid?) and young Joe go from the house to the barn through some underground tunnel. When Jesse leaves, they open up the cover of the tunnel and both come out and enter the house. That was risky because Jesse might have seen on rear view mirror or Old Joe might have arrived there, so it was safer that Sid and young Joe would have gone back from the barn to the house retracting the path of the underground tunnel. As such, they needn't have come all the way to the barn through the underground tunnel. They could have as well remained below the house where the underground tunnel starts.

It has not be made clear why they had to send silver or gold bound to the body of the victim. Converting so much silver or gold repeatedly to currency of the current time would have raised suspicions of current law-keepers. They could have sent silver or gold separately to Abe who would have converted the same to current currency and would have paid the killer Looper after the kill has been confirmed. This way, there would not have been current legal issues and the payment gets made only after job-done.

Young Joe suggests that the crop be burnt to get a clear view of Abe's man or Old Joe approaching. Sarah refuses saying that is Corn seeds that have to be kept for next year's crop. Later, when the kid kills Jesse and young Joe shoots at Old Joe and then Sarah, the kid and Young Joe are in the crop field, and there is a wide view of field, not a single corn is visible on any of the plants.

In a scene on the steps at Sara's house, Joe calls Sara "Emily".

Young Joe tells Sarah in no uncertain terms that Abe's gang or his loop or both would arrive in 15 minutes so she had just that much time to take Cid and run far away. Then, Kid Blue captures old Joe and takes him to Abe where all that fight and killings takes place, then old Joe escapes, then young Joe kills Kid Blue after all that shooting, then young Joe takes the van and silver and reaches Sarah's farm, all this must have taken some hours. But, that 15 minute of deadline meant nothing for Sarah and she was still packing things after all these hours are gone by, and leaves just in the nick of time when both Joes arrive.

When Cid and Sara speed towards Old Joe and the car flips and lands on the road, a roll cage supporting the car's roof is visible.

The crop on Sara's farm is sugar cane, which grows in a hot, humid climate like Louisiana, where the movie was filmed. Her farm is supposed to be in Kansas, where sugar cane isn't grown.

Gold is one the higher density material, with 19.32 grams per cubic centimeter. A gold bar weighs some 12.4 Kg. Silver would be in similar range. All the movies showing briefcase full of gold being carried like it is full or cotton buds are incorrect as a normal person just cannot even lift briefcase full of gold. With so much gold or silver tied to a person's body, he would not have been able to walk, yet, old Seth and Old Joe have reflexes intact and they run away like they are not having an extra gram on their bodies.

The premise of the whole movie is that in some unexplained way it is difficult to get rid of bodies in the future, so they send the targets back to be executed and disposed of. Surely, it would be less risky to execute the mark in the future and just have the team in the past do the disposal, which would prevent any scenario like the one in the film occurring.

A clever scene had old Seth losing parts of his body as his decades younger self was surgically getting amputations as he desperately tries to reach an address that is scarred in his arm. But this was happening in a vacuum, without Seth being affected by the history of missing those limbs. I.E. if young Seth lost his leg, by the time he became old Seth and was rushing to get to the address he would have a prosthetic leg. He just becomes this multiple amputee with no butterfly effects to his history.

All these deviations arise because they were sending the older Looper to get killed by his own younger self. Had they been sending to some different Looper, there would not have been any psychological issues, and no need to pay in gold instead of usual silver.

It's stated that it is impossible to kill people in the future, hence the need to send people back in time; however, Old Joe's wife is shot in the future. It is never explained.