There's no way to fix this except replace the LCD. I've have something like this happen to my HP netbook.
The cause of the white spot is that you damaged the backlit film that spreads light evenly across the panel. When you apply pressure against the back, this distorts the plastic film and makes the affected areas whiter.
This could have happened if you put the n3DSXL in pants with tight pockets. Walking around with it on you is enough to push against the back. From now on, carry your n3DSXL in a hard case and don't stack heavy objects on top of it.
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SylverReZ:
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Psionic Roshambo:
