A number of users of Nintendo’s 3DS have been reporting issues with their top screens becoming scratched with no external cause. It was first reported on the gaming forum Neogaf, but has since become far more widespread and a simple google search for “scratched 3DS screen” will show. Nintendo has yet to comment on this but retailers are aware and some are replacing the damaged units.
It appears the hinges on some units are not aligned properly and are causing the groove on the lower screen to create scratches on the upper screen. The scratches seem to also affect the 3D aspect as well, as it breaks the illusions as your eyes try to incorporate the lines into the 3D.
Have you been having any problems with this? My 3DS shows no signs of this yet, but perhaps once we hear official word from Nintendo we will see if there was specific lot of them that were manufactured improperly.