I recently wrote a library that rasterizes a PDF page to an image and was observing a very similar behavior. It was also highly system dependent and erratic as well. I would randomly see a distorted image of my own screen when I run my program on any PDF. Then figured out I was not initializing the buffer I was using to render the PDF page to image. After I memset'ed it to zero, the problem went away. I still couldn't quite comprehend what was exactly happening in the background.