The one passage that I can remember distinctly showing Severian's status as an 'unreliable narrator' is from The Claw of the Conciliator when he's speaking about his eidetic memory:
"The truth is that I am one of those who are cursed with what is called perfect recollection. We cannot, as I have sometimes heard foolishly alleged, remember everything. I cannot recall the ordering of the books on the shelves in the library of Master Ultan, for example. But I can remember more than many would credit: the position of each object on a table I walked past when I was a child, and even that I have recalled some scene to mind previously, and how that remembered incident differed from the memory of it I have now."
He admits that sometimes he recalls the same events differently, which calls into question the rest of his narrative. He also mentions somewhere that he believes he is going insane. Those were the two examples that jumped out at me during a first reading of the series.
His narrative can also be questioned due to the lack of details he gives for major events that take place during the story. It's been a few years since I've read through the books and I can't remember the specifics, but I always felt that Severian was intentionally omitting certain details in his memoirs. This leads the reader to question what it is he's hiding and how much they can actually trust to be the truth.
Edited by duckman, 29 April 2008 - 02:10 PM.