Different memories
The movie is a combination of the action that led up to the library robbery and interviews with the actual individuals who did it. While the real men who went through with the robbery are re-telling their stories, there are clashing memories between them - Warren and Spencer in particular. The examples include the question of who proposed the idea first, what was the man who was a link to sell the artwork wearing, and finally whether Warren made the trip to Amsterdam to negotiate selling or not. It is interesting and ironic because it provides a sense of unreliability in the story.
Planning the robbery vs. actually doing it
While they are planning the robbery, the four students are feeling excited and even delusional on how it’s going to go, fantasizing about it as a scene from a movie. But, when it comes to actually doing it, they realize the reality and consequences of it. Warren, especially, gets a taste of reality while dealing with the librarian and seeing his actions hurting another person. Nothing goes according to plan, they frantically escape with a fraction of what they meant to steal. Even as they try to sell it, their stupidity is shown, and it eventually leads to their arrest.
The immediate regret
The students are immediately faced with reality after they go through with the robbery. This extraordinary and exciting thing that they planned to do, and live out a special and carefree life after it, crumbles down. There is a sense of immediate regret, and their dull and meaningless lives suddenly seem very appealing, and losing those lives seems devastating. This is shown with Warren’s heartfelt conversation with his father and Spencer regretfully watching his family celebrating his father’s birthday.
A line crossed
Despite it being a cautionary tale, and a tale of redemption, the person whom the students hurt the most - the librarian Betty Jean Gooch, makes an important point about their actions. She describes them as very selfish and notes how they crossed a line where they justified hurting another person for their own benefit.
