Apple has started seeding the first beta of OS X 10.9.5 to developers. The code is listed as built 13F7, and available through the Developer Center. Users are asked to focus testing on just a handful of areas, including Safari, graphics, and USB, including smart cards.
The company has simultaneously posted the first betas of Safari 6.1.6 and 7.0.6, for OS X Mountain Lion and Mavericks, accordingly. 6.1 and 7.1 betas are actually available already, but Apple appears to be preparing interim updates to deal with site and extension compatibility issues.
10.9.5 may be the last major update of Mavericks. OS X Yosemite is due to launch in the next few months, and Apple typically winds down support of earlier operating systems as new ones emerge.
