Johnson covers up the potential issues well enough, but there are places where the film asks us to blindly accept a confusing logic that relates directly to the major, major events of the film. Can the events depicted actually alter the future? How much interaction does it take before the butterfly effect takes hold and all possibilities are destroyed? If you know (essentially) the day you're going to die, why don't more loopers disappear? How many people in the past are aware of time travel in the future? Doesn't that awareness impact its future use? Also, did they really have to mess with JGL's face that much to make him a convincing Willis stand-in? My advice to you: as long as it's not wondering why JGL's lips look so made up compared to Old Joe, don't think about it. Just go with it. There's enough substance to the story outside of the scientific logistics and Looper uses its assets to pull the story in truly unexpected directions far away from the average formula.