The 15" was not updated. If you look at the days since it was last released, it's due for an upgrade soon. Maybe they have something bigger for it planned or they're waiting on intel for some new chipset.
It does it in both mirror and extend. I've just determined that it's happening when the laptop is using the discreet graphics card, which it does whenever powering an external display. I'm not sure why it was doing it over airplay, with graphic switcher it's not switching to discreet and therefore not showing the glitches.
Just installed Mavericks on my wife's computer (2012 non-retina MBP). She has an external Dell display connected over mDP to DVI. When it's connected it gets this crazy artifact thingy. Anyone seen this?
It does it on both displays equally, but goes away when the exernal display is disconnected.