Hardware fails sometimes. Hell, the original Game Boy I had had the screen lines going off after two years of not being played. (It's an easy fix, just melt solder onto the lcd screen points.)
I've never had a problem with the original 3DS (Cosmo Black) that I bought back in the day. Hell, the corners of the system were damaged but the ribbon cable was ok. I've since owned a LoZ 3DS and a N3DS and there were no problems in either of them.