In a stapled binding, unplated steel staples can rust in a humid or damp environment. The resulting iron oxide can cause brown staining of the wraps and inner leaves. In this damp environment, the paper may also acquire a musty odor. In the worst cases, the staples may rust to the point of disintegration. See Staples, Rusted.
The first image shows books with light-to-moderately rusted staples. The examples to the right have wraps that have been stained by the rust. The second image shows rusted staples that have stained the book's leaves in the gutter of the gather.