System Information
Win8.1 64bit { GTX680

Blender Version
Broken: 2.71.6 a0b3604

Short description of error
In UV/Image Editor, Alt S cannot save image, which reports:"Cannot save image, path ' ' is not writable"

Exact steps for others to reproduce the error
Open image or just render the scene, then Alt S in UV/Image Editor.

PS: I wonder if this is the same bug as this one that revived again? 2.70a also has this issue as I just tested. Thanks for checking.