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 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: http://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: $\displaystyle\sum^N_{n=1}\left(\prod_{i=1}^n b_i \right)^{\frac{1}{n}}\le \sum^N_{n=1}\left(\prod_{i=1}^n a_i \right)^{\frac{1}{n}}$?