I was playing halo2 campaign on my 360 and I noticed that some images get burned into the screen. I snapped pictures of the arbiter's silhouette, burned into the left side of the screen. One of those bigger forerunner robots also got burned into the screen.
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I am not sure about you guys, but my 360 used to do this a while back, but lately it has not done this at all. Maybe, just maybe, after a while of playing, the TV and the 360 become immune to the defect that you are seeing.