On the Kindle 2 and DX, some MOBI ebooks take advantage of navPoints (waypoints) to provide additional navigation options. These seemed initially to be invisible except on the Kindles, but they do have a very small effect in Windows MobiPocket Reader. Normally the book title is shown at the top left (above BOOKSHELF), but when waypoints are present this is the title and chapter. See the enclosed screenshots. The first is from the original version of
Ring of Fire by Eric Flint, and the second after running the MOBI through Calibre 0.6 which adds waypoints.