Mac OS X 10.9.5 MacBook Pro Retina, Mid 2012
Broken on Blender 2.74 and 2.75 both CPU and GPU (Nvidia GT 650M) rendering
Open the file and keep pressing Shift-Z to switch between rendered and solid viewport. For me, image is sometimes rendered as horizontal strips and sometimes as a grid.
It looks like alpha channel of images that have Use Alpha unchecked, although always rendered white when connected directly, somehow influences the mix node in a strange way.