I'm not sure what setting or anything has caused this, but when I resize an image in
Image > Image Size, the resized image gets semi-transparent 1px border! There seem no obvious settings to cause something like this.
It does'nt seem to do it 100% of the time, I notice it more when I'm cutting up a design and paste images into new documents. Here I have recreated it with a simple 2-layer image: