There is a thread is GQ about the “fourth wall” in drama. This post by Stranger On A Train referred to the Unreliable Narrator in mystery stories.
What is it? Is this a narrator that misinforms the audience to mislead them about the resolution of the mystery?
Examples please?
I haven’t heard the term before, but a bit of quick googling suggests that it’s talking about first-person POV, where the character who is narrating the story is clearly biased enough in their point of view that what they are telling, the way in which they describe things, is not an objective description of what’s going on. This could be very minor, or extreme, (the main character having involved hallucingenic episodes which he describes as reality,) and may or may not become clear only afterwards.
Clearly, it’s a good trick for the author to signal that his narrator is unreliable when the only channel of communication he can use to the reader is through that narrator. The Jeeves books might be a good example, where the narrator is an idiot and consistently misreading situations, (IIRC,) but you can see immediately that the author is a genius, precisely because of how well he writes that guy stupid.
One example of the unreliable narrator in mystery stories is the narrator in Agatha Christie’s The Murder of Roger Ackroyd. I will say no more. 
An unreliable narrator doesn’t have to lie – he can just leave out relevant information. And he can be speaking the truth – as he sees it – but the reader can pick up on the idea that the narrator is missing things. He may talk about how his wife and his best friend often go to lunch together, for instance. Nearly every day. Long lunches. He’s so glad his wife has something to keep herself occupied while he’s at work . . .
Here are a couple of good sites that include definitions of this term:
Wikipedia on Narratology
A Glossary of Terms Useful in Critiquing Science Fiction
From the latter:
Another example is Lawrence Block’s WHEN THE SACRED GINMILL CLOSES. The story is told first-person by the detective, who reports accurately on the case, but is unaware of his own situation … that he’s an alcoholic Makes for a very, very powerful story.
Archie Goodwin, who narrates the Nero Wolfe mysteries, from time to time withholds information from the reader. He’ll say things like, “I won’t tell you what was in the envelope, not just yet.”
For my money, one of the best instances is the William Holden character in Sunset Boulevard when we learn at the end that he’s dead and he’s been telling the story from his POV all along! I’d call that unreliable – in spades.
Adrian Mole is very unreliable in the sense that he’s clueless to what’s going on around him.
I’ve never seen it, but don’t we know that he’s dead from the very beginning? I understand that the first shot is him, face-down in a swimming pool?
My own nominee is The Strange case of the Dog in the Night-time, in which the narrator is autistic (or nearly so).
A rather famous example is Fight Club where Where the Narrator doesn’t realize he’s also his anarchist best friend until near the end.
It’s been quite a while since I saw it, too. My memory is that we do see the body face down in the pool at the beginning of the movie, but it’s only toward the end that we learn it’s Holden’s character. I guess I need to see the thing again soon. Anyway, there are other movies where the narrator winds up dead. There are even movies like Casino where different characters wind up narrating. That has to be suspicious, if not unreliable.
I seem to recall an SNL skit that’s a ripoff of David Copperfield (I think) where the first chapter is “I Am Eaten By Sharks.” That got a giggle at the time.
I wouldn’t call it a twentieth-century development. Poe and Dumas used unreliable narrators.
I think Phillip K Dick is the king of unreliable narrators.
A Scanner Darkly, Do Androids Dream of Electric Sheep, Ubik,
A classic movie example would be Memento. Do you trust the guy who has no memory? Or do you trust Teddy, the guy we know is a liar?
I just watched Sunset Blvd this weekend. It’s not the first shot but we do see Holden in the pool. Both face down, and then a dramatic shot from underwater where we see his face and the police looking down at him.
We did a section on unreliable narrators in one of my college English classes. Some short stories to check out:
“The Yellow Wallpaper”, by Charlotte Perkins Gilman
“Rape Fantasies,” by Margaret Atwood
I have a further question - what do you call it when you follow a character around in the third person, and yet are left with just their feelings and ideas about the situation? The Unreliable Narrator’s Friend? 
the Handmaid’s Tale by Margaret Atwood employs and “unreliable narrator” device, made all the more interesting in that the narrated story has an epilogue to it, in which the story is critically analyzed by a future (fictional) professor who points out the contradictions in the narrator’s story.
Thanks to Zebra for clearing up my poor memory of SB and Holden’s character. I must have been paying less attention when I saw the movie. I just remember thinking how weird it was to be hearing the story from the POV of a dead man.
In a slightly different slant on the OP’s issues, wouldn’t Verbal Kint in The Usual Suspects qualify as an Unreliable Narrator?
And did any of you recognize Spacey from that fax picture? I didn’t. There was a similar problem for me in Jagged Edge trying to identify Jeff Bridges. It may just point to my poor eyesight, but when these clues are all we’re given I miss them sometimes. I’m claiming similar problems in recognizing Holden’s face early on in SB.
I am not sure that Verbal Kint is an unreliable narrator in the classic sense, because I think he is knowingly telling a false story. I think an unrelaible narrator must think they are telling the truth, but we can’t discern the truth from the fiction. Verbal Kint we know is full of it and nothing in his story is correct.
A better examble is probably Leonard from Memento and his story about Sammy Jenkis as well as his recollection of what happened to his wife.
FWIW, the Unreliable Narrator is not a device of “mystery” literature; it’s a standard device used pretty widely across all genres.
My favorite example is The Dwarf, by Par Lagerkvist. The Dwarf consists entirely of the first person ravings of a psychopath. We are able to see through his self-serving, twisted view of reality to see that HE is the cause of all that goes wrong around him, even though all we ever hear is “his side” of the story. Every story he tells, he tells in such a way to place blame on others, and put himself in the best possible light, but Lagerkvist still manages somehow to make it pretty clear what’s really going on. In other words, the narrator is passing us information he himself is utterly unaware of; in fact, he’s trying to feed us an entirely different agenda. They don’t call it the Nobel Prize for nothin.
Gene Wolfe’s narrators in his New Sun and Long Sun books are notoriously unreliable.