I came across this problem after printscreening a website with a gradient background. Initially, I was shocked to find that the background was showing those black pixels, and thought something might be wrong with my GPU so I decided to investigate a little and found out that the image's bit depth was 24 and photoshop was opening it in 8bit mode (maybe because photoshop doesn't have a 24bit mode?).
I was wondering: How come when I save the image in 8bit mode it doesn't show those black pixels, in programs like windows photo viewer and paint? Why are they showing only in photoshop? Shouldn't photoshop find comparable colors to fill in instead of black?
When I change the mode to 32bit those black pixels disappear. Also, when sampling those black spots with the eyedrop I am getting the correct color.
This is the image I was speaking about:
And here's what I am seeing: