LCD's don't have screen burn... that's plasmas. Are you thinking of ghosting?
no but wats ghosting?
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ghosting is when the screen refresh rate is too low (say you have an older LCD, or a really cheap new one) for whatever you're doing. You usually see ghosting most on older LCD's when you're playing graphic-intensive games. Ghosting looks basically like there's a ghost image (trails) of whatever's on your screen.
Or... here's wikipedia's entry for ghosting (regarding lcd's)
"A problem in LCD screens when tiny pixels creating the image take time to switch on and off and can't do it fast enough. The problem, widely recognized as the main drawback of LCD screens, is apparent in fast moving objects such as tennis balls, but even slower moving images get fuzzy. This can happen especially when the device is exposed to cold for long periods of time. This problem is usually not apparent on modern LCD screens."