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I've experienced things like that quite a bit - the culprit is usually Windows handling portrait-oriented monitors poorly, and a restart and/or graphics card driver reinstall usually takes care of it for another month or so.
The one I get most frequently is a few miscolored pixels, usually in a line, that follow my cursor around the screen. It's annoying, and I usually put off restarting until I just can't take it anymore, but it has never been anything that a good restart didn't fix for me.