I have started having a problem when I expand the canvas on an image.
I use the crop tool to outline the existing image, pull the border out where I need to add canvas, hit enter.
I then use the marquee tool to select the area of the original canvas that I want to expand into the new area created by the crop. It works well, except when I hit command D (mac), I am left with a razor thin parting line at the point were the marquee tool selection box started. This would be not on the outer edge of the canvas I selected, but rather on the inner edge (towards the main body of the image).
The line is very thin, but distinct. I didn't originally have this when I started using this tech., but can't figure out what is different in my settings or tech. that might be causing this.
As far as I know this is a Photoshop bug and only happens in 16 bit. I use the rectangular tool to select the area and then the patch tool to remove it.