Minor Plot Hole: As the plot tells us, getting rid of bodies in the future is next to impossible… unless they’re of Chinese women in small villages, then you can just shoot them and burn the corpse in a house fire. For everyone else though, you need hyper-illegal extremely risky time travel operations.
Major Plot Hole: Why do the Loopers have to kill their own future selves? The Looper program is shown to have at least a half-dozen Loopers, and probably has a lot more. While forcing your past Loopers to kill their future selves is suitably sadistic for future super evil crime lords, it would be a lot more efficient to just send them to other Loopers, who probably won’t notice that they’re killing the future forms of their friends. That also means you could just keep the Loopers in the dark about their ultimate fate, which would make them that much more obedient. If the future super evil crime syndicate leaders haven’t thought of this, then I’m going to guess they’re all comic book super-villains. Which would be awesome if they also have super-telekenesis (POST-HOC SPOILER WARNING!).
Major Plot hole: Old Joe’s plan for survival was pretty stupid. He sent himself back in time without a hood and just assumed his younger self would not shoot him. He could have put a body armor on, or at least put the gold case on his chest instead of his back, or put some Vaseline on his bald head to reflect some manly light on his younger self. But he did none of that. The point is that Old Joe got incredibly lucky to survive an ordeal he knowingly put himself through without even altering his potential death trap with the shadow of a strategy apart from the ‘’Gee, I sure hope he recognizes and gives a shit about me before the 0.5 second it usually takes him to shoot his targets’’ plan.
Super Plot Hole: The very concept of Looper executions is bad for your brain cells. The movie’s plot explains that, in the future, disposing of a body is exceptionally difficult, but there’s no mention of any difficulty associated to killing people. Abe then goes out of his way to explain to the audience – I mean, to Joe – how dangerous it is to the future to have someone from the future running around the present. Given the risks, why don’t super-evil-dystopian-future-crime lords just have their henchmen execute their victims in the future and then send back a corpse? Since this isn’t a zombie movie, corpses can be assumed to be much less of a flight risk. It would probably also be a lot cheaper. This could be easily justified by saying only living bodies can travel through time – for some reason – but the movie apparently cut those three seconds of dialogue in favor of showing gratuitous stripper tits. And for that: we say thank you.
Super Plot Hole: Why even have Loopers at all? Each Looper is established as having his own designated execution spot, which doesn’t have any specific preparation except for putting a little plastic tarp down. So if the future evil crime time travel machines can drop their victims anywhere on Earth… why not program them to drop their victims at the bottom of the ocean, inside a volcano, over a pit of sharks with frikkin’ laser beams, or some other inescapable death trap? Apparently, the super evil crime lords of the future picked a complete idiot to run their Looper program, since Abe decided the best way to dispose of people was to have idiot junkies kill them alone out in the middle of nowhere without any way to definitely guarantee that the victim has been actually killed.
Plot contrivance: Time to get SUPER DEEP. If you already feel dizzy skip this one altogether. Even if time travel is all fiction, you have to obey your own rules because if you don’t – it creates what we call a ‘’technical clusterfuck’’ that doesn’t make any sense when you try to decipher it. Here we go.
On one side, you have the alternate timeline mythology of the screenplay where Old Joe’s past reveals that young Joe killed old Joe when he was young. He got his gold and lived happy for 30 years before being sent back in time. At that point, Old Joe knows about his fate and decides to alter it by surviving his execution. This creates an altered timeline both different and independent from the previous one. Indeed, if it was not independent, Old Joe would have been unable to change his fate; he would have been killed no matter what he tried to do. So the movie firmly established that you can alter your fate and create a new alternative timeline for yourself.
On the other side, you have aspects of a single loop-timeline mythology where what happens to young loopers also happens to old loopers. So for example, when young Seth is tortured and his limbs are cut off, older Seth in the present also gets scarred.
But this doesn’t work. If young Seth gets one finger cut-off, he will grow older missing one finger – let’s call him Old Seth B. But Old Seth A from the original timeline who is right beside him in the present should not be affected by that because he doesn't belong to this new future anymore. It is confusing to see both the young and old versions of Seth as the same person. At this point in time, being in the present together, they should be seen as brothers. If one gets killed – it’s sad, but the other older brother doesn’t get physically hurt. He is not in the future anymore – he is independent from whatever happens to this man, because his own future was dictated by his own younger self version that stop being his version the moment he altered the timeline by surviving his fated execution. In short, whatever happens to Young Seth B will affect Old Seth B, but the Old Seth who altered the timeline and is living in the present now is not Old Seth B, it’s Old Seth A.
TL/DR: Young Seth gets his limbs cut off: nothing should happen to Old Seth. Young Joe kills himself: nothing should happen to Old Joe, he would just be amazed by the gutsy and stupid move of his younger self and would proceed to kill Cid's mom.
Plot contrivance: Why send the loopers back in time 30 years after their last respective killings of their respective future selves? Why not kill them 5 years, or 6 months, or just a week after they killed their last targets? Surely keeping them alive for 30 years without any supervision and monitoring is the best way to make them spill the beans and talk to everyone about their dark pasts as soon as they get drunk.
Plot contrivance: Why not hire old guys to be loopers? They would only need them for a couple of years and they would all be dead before 2074 – negating the need to send them back in time to get killed as mother nature would have already taken care of that by nicely elbow dropping them in the face with all her grace.
Plot contrivance: Yes, Joe heroically kills himself at the end of the movie to make sure Cid doesn’t grow up without his nurturing and loving mother – but that was the case in the original timeline as well. Cid pretty much becomes an evil telekinetic adult douche-bag with or without his mother’s involvement. So truly, as depressing as it sounds, Joe probably killed himself for nothing. High Five!
Unaddressed Issue: This flaw is inherent in most time travel movies but here it goes: If time travel is possible in 2074, does that mean it is also possible in 2134? And 2567? And 3452? How come Earth is not constantly bombarded with time travelers from an infinity of future years? Why are there time travelers ONLY from 30 years in the future and nothing after that? Was it just a fad?
Unaddressed Issue: So bad guys invented time machines. That’s cool. Are they using them to go back in time and become rulers of previous eras? No, ok. How about going back in time to win the lottery Back to the Future-style? No, ok. How about using them to meet famous people who might possess deep secrets about humanity like Jesus, Julius Caesar or Patrick Swayze? No, ok. So those guys created time machines and the only use they could come up with was dumping people 30 years in the past to get killed? That’s a little underwhelming - even by sci-fi masturbater standards.
Unaddressed Issue: How does Sara know about Loopers? Joe seems surprised that she knows about them, meaning they’re probably a well-kept secret. He even asks her about it, and she never answers.