It's not just a hunk of cork that doubles as a 3-D Pinterest; it's also a symbol that packs a wallop. In fact, the bulletin-board-as-symbol blows up the whole plot of the movie…not to mention blowing thousands of mid-90's moviegoers' minds.

When Kujan finally notices the details of bulletin board, he discovers that Verbal made up his story…using details on the bulletin board. Huh. Maybe this is why interrogation rooms are typically devoid of paperwork and inspiration boards?

After all, Sgt. Rabin describes his office (including the bulletin board) as a place of controlled chaos:

"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..."

The bulletin board itself symbolizes the untrustworthiness of certain stories, and how narrators sometimes have their own questionable motives. Even the company that made the bulletin board—Quartet in Skokie, Illinois—becomes the basis for a Verbal lie. In fact, it's this detail that tips off Kujan.

Earlier in their conversation, we hear this exchange:

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. 

And this bit of info is relevant…but just not for the reasons Verbal states. It's relevant because once Kujan sees that the bulletin board was manufactured in Skokie, he realizes that Verbal was using visual cues to piece his story together.

And that he used a lot of them. From the bulletin board we realized that Verbal's story about picking beans in Guatemala was bogus and that the name Redfoot was totally made up.

Kujan learns that Verbal's story doesn't point towards Keaton—it points toward this bulletin board…which points toward Verbal as the true Söze.