I've updated x11-drivers/nvidia-drivers from 270.41.19 to 275.09.07, and I found a bug whenever a window smaller than 32x32 shows up. Basically, instead of the window contents, all I get is a glitched image, probably from somewhere else in the video memory.
Here is a video showing the bug: http://www.youtube.com/watch?v=ZeZZy7QNjI4
I haven't tested 285.05.09 (yet).
It seems that version 285.05.09 works correctly.
I've also posted this bug at my blog, like I did with all previous nvidia bugs I had: