Seems to happen at the end of builds when a material is applied to a landscape and built on lit mode. Building on unlit mode or having no material applied does seem to work but it intermittently crashes when switching to lit mode with an applied material.
I've got January (2012) UDK crashes too, but in different case.
Crashes allways appear when material window is maximized after minimizing (udk crashes not every time but often). Also framerate in perspective viewport slows down a lot when material window is minimized (and you know that it will crash if you maximize windows back). The same thing appears even if I just open material from content browser, and it's minimized from the start.
There is a way to prevent crash: if close minimized material, framerate rise up and you can keep working 'till next "pre-error case".
Tested on different maps with different materials.
Jan'12 build (x86 and x64). Windows 7 SP 1. DX 9 mode.
I am also having my sound occasionally cut completely out in Play-in-Editor with January 2012 UDK. The only way I can get sound back is to close the editor, reopen the editor, and reopen the map and hope it doesn't lose sound again when I go back into the play-in-editor window. (which is hit-and-miss) I have never had this problem with November 2011 or December 2011 UDK.
At home Im using october public build to avoid the latest bugs. With licenseed versions happens too. Today I switched to dx9 at home because cubemaps are not working in dx11, and yes, it crashed every time I tried to maximize someting. So I would say its a DX problem. Seems to work PERFECT with Dx11.