From the initial preview to its final release, Mountain Lion took a journey that lasted over five months and saw 269 builds of the update before finally going gold in July of 2012. In fact, it was more than a month after its big WWDC keynote that Apple finally got OS X 10.8 out the door. But, things may be a little different this time around. Bug reports are starting to pop up in Cupertino's Radar system with the operating system build number 13A451. The first half, "13A," strongly suggests platform in question is OS X 10.9, since Mountain Lion was 12A, Lion was 11A, and so on, and so forth. The second half implies that this is the 451st build of the (not so) mysterious OS known only at 13A. That means that there have already been almost twice as many internal releases of 10.9 (presumably Cougar or Cloud Leopard... or maybe Scottish Fold?) as there were of 10.8 at the time of its release. This could indicate that Apple is preparing a substantial overhaul of its desktop OS, or it could just be that the company wants to ensure it avoids the usual flood of issues that accompany upgrades to the operating system. Either way, with WWDC right around the corner we're sure the mystery will be solved soon enough.
Source: Apple Insider