VERBAL: Back when I was picking beans in Guatemala we used to make fresh coffee. Right off the trees I mean. That was good. This is s***, but hey, I'm in a police station.
This is a pretty innocent comment. It doesn't seem to have much importance. But it's not actually true—it's a detail that Verbal/Söze picked from the bulletin board in the police station office to concoct his story. Kujan notices it and the other details at the very end, and realizes what a mistake he's made.
KUJAN: A rumor's not a rumor that doesn't die.
Kujan thinks that there must be some truth to a rumor with staying power. Unfortunately, he forgets this himself, imagining that Söze doesn't really exist, and that Keaton was behind the caper.
VERBAL: A man can convince anyone he's somebody else, but never himself.
In retrospect, this takes on deeper meaning. Verbal convinces Kujan that he, Verbal, is really the man he claims to be—when he's actually Keyser Söze. His confidence in his true, evil identity makes him a pretty cool customer.
KEATON: His name is Verbal. Verbal Kint.
VERBAL: Roger, really. People say I talk too much.
Verbal talks too much—and he also speaks in wild, improvisatory lies that totally convince Kujan. Since Kujan thinks Verbal talks too much, he thinks Verbal is revealing things about Keaton that he doesn't intend to reveal—but that's exactly what Verbal is trying to do.
VERBAL: That was how I ended up in a barbershop quartet in Skokie, Illinois.
KUJAN: That is totally irrelevant.
VERBAL: Oh, but it's not. If I hadn't been nailed in Illinois for running a three card monte in between sets, I never would have took off for New York. I never would have met Keaton, see. That barber shop quartet was the reason for everything.
As Keaton discovers at the end, this barbershop quartet was just something fabricated from the label on the bulletin board, which was made by a company called Quartet, in Skokie, Illinois. So, even if it's not the "reason for everything" it is a clue that tips Kujan off—all too late.
KUJAN: Dean Keaton was dead. Did you know that? He died in a fire two years ago during an investigation into the murder of a witness who was going to testify against him. Two people saw Keaton enter a warehouse he owned just before it went up. They said he had gone in to check a leaking gas main. It blew up and took all of Dean Keaton with it. Within three months of the explosion, the two witnesses were dead, one killed himself in his car and the other fell down an open elevator shaft.
This is evidence that Keaton is a deceitful and malevolent dude. It contradicts the rosier picture of Keaton that Verbal tries to paint.
KUJAN: It was his idea to rob the taxi service back in New York, wasn't it? C'mon, tell me the truth.
VERBAL: It was all Keaton! We followed him from the beginning! I didn't know! I saw him die! I believe he's dead, oh Christ!
After we learn that Verbal is Söze, we realize that he's deceiving Kujan here by pretending to have been deceived by Keaton!
RABIN: Yeah. It's got it's own system though. It all makes sense when you look at it right. You just have to step back from it, you know? You should see my garage. Now, that's a horror show...
Rabin comments on the disorder in his office, and speaks truer words than he realizes. The "system" Kujan missed seeing was right on the bulletin board—the source of all Verbal's extravagant lies.