Honesty comes into the picture even while writing a story. If you're writing from the point of view of a character, you can't suddenly reveal a truth that only she knows but doesn't reveal until the end of the story, for the twist in the tail. Sounds like an obvious point but it isn't.
Here's an example, from a prize winnning story I read somewhere:
The story is written from a woman's point of view, let's call her Ann. A police officer comes to Ann's house and tells her her husband has been killed in a distant country and that a foreign woman is claiming to be his wife, and is expecting his baby. A DNA test is decided upon to clear matters. The lineage of Ann's son and the new claimant's baby will be checked. After days of tension, it is revealed that the foreign baby and Ann's son do not share the same paternal DNA. Ann's family is relieved, but the claimant is distraught because she has proof of the marriage and is adamant that it is indeed his baby. At this point, Ann casually tells the reader that her son was actually fathered by her husband's friend--she was not faithful to her husband! This is why the DNA doesn't match. She tells no-one about this and thus ends the tale, so her family's honour in place.
It's a clever plot, but it hid the truth until the end, keeping it like a trump card. If it were revealed at a natural point in the story - like when the DNA matching is suggested--it would have been honest. The reader feels conned by such a story. A good writer does not con the reader in this obvious way--she has to do it well!
Story writers have to be aware about this when considering the point of view to tell a story. Use the unreliable narrator, but do it so that your story remains believable. In my story 'Brownies for Christmas', too, I use an unreliable narrator. A woman is narrating the story, about making brownies for her daughter who is suffering from brain cancer. She talks as if her daughter is alive, but the reader learns the truth through her husband who reminds her that their daughter is no more. This is the twist in the tail.
The difference between this and the previous story mentioned is that this narrator has not come to terms with the fact of her daughter's death--she truly believes that her daughter is alive. The truth is revealed towards the end but it is not withheld knowingly, and this makes all the difference.
If your twist depends on a revelation, make sure that it happens in a believable and honest manner.