LCD monitors can have "dead pixels" which are pixels that are stuck at one color. This can be a manufacturing defect in the monitor (an artifact of the extremely complex technology used to make these things), and you can't fix it, at least not that I know of. Most monitor companies have a policy whereby you can return and exchange a monitor with a certain number of dead pixels. The number of dead pixels where they'll let do that you varies by manufacturer; my monitor it was 5 or 6 I think. If you just bought the monitor, I'd check to see if you can exchange it.