To some extent, all first person narrators are unreliable. After all, they're recounting events filtered through their own unique set of experiences, beliefs and biases. There isn't just one absolute experience of reality. A first person narration will be shaded by everything that makes that particular character unique and individual. This is true of even the most honest and objective personalities.
One narrator might see a charity gala as an overly decadent affair because he spends a lot of time in the poverty stricken village the charity serves. The contrast between rich and poor is sharper to this narrator as he considers that the money spent on just one of the plates for this event could have saved the life of a young mother who was not able to receive the medical attention she needed because she could not afford the trip to the only area hospital, hours away by bus. Another narrator, one of the organizers, might describe the gala as spectacular and moving. She focuses on the more general picture—the school that will be built and staffed as a result of this fundraising effort. Is the event gaudy or glorious? Perhaps it's both. There's no one right answer.
Of course, there are certainly narrators who are more unreliable than others. They may misunderstand or misreport events, leaving readers to make their own judgments. Holden Caulfield in J.D. Salinger's The Catcher in the Rye is unintentionally unreliable. His youth and inexperience often obscures his full understanding of some of the people he meets and situations he describes. The narrator in Edgar Allen Poe's short story “The Tell Tale Heart" is mad, leaving the reader to sort out delusion from reality. In Vladimir Nabokov's novel Lolita, Humbert Humbert often describes scenes in a way that justifies his sexual desire for twelve-year-old Lolita. Humbert Humbert, no doubt, believes this version of events, but the reader can see beyond his perspective.
Unreliable narrators can create intrigue. A narrator that seems reliable may reveal details that make the reader question his credibility. This can become an evocative source of tension. Unreliable narrators can also make for complex characters. Readers may delight in discovering the reasons behind the narrator's lack of reliability and going deeper into his peculiarities and motivations
Some people simply cannot (or choose not to) be on the up and up. This is another facet of the human experience that fiction can explore.