Maybe it could be called book-errata?
I cannot give many examples of discussions from math.SE offhand but at least one example from here https://math.stackexchange.com/questions/34641/find-limit-of-unknown-function
This is an example from a different forum: http://www.sosmath.com/CBB/viewtopic.php?p=181367
I guess questions (and answers) revealing mistakes in book will appear here occasionaly. (Perhaps even withou this being the original intent.)
A related (interesting) link: https://mathoverflow.net/questions/3038/errata-database/3040#3040
Although I have tag-creating privileges, I've never done this before and I wanted to ask about opinion of other members first.
EDIT: Here's a recent post of this type: Showing $\sum\limits^N_{n=1}\left(\prod\limits_{i=1}^n b_i \right)^\frac1{n}\le\sum\limits^N_{n=1}\left(\prod\limits_{i=1}^n a_i \right)^\frac1{n}$?