When I try to play DS games on my 3DS, they boot up fine (some games require a little wiggling and dust-blowing to be recognized, but all games did on my DS Lite anyway), but the touch screen doesn't respond at all, even though other controls work. When I exit the game and go back to the 3DS home screen, the touch screen is completely fine again.
When I try to play the DS game anyway, at complete random, the volume will become blaringly loud, even when I turn the system's volume slider all the way down to mute. This seems to happen any time from immediately upon booting to several minutes later, and seems to happen a bit more readily if I close the screen partway or start up the game with the system muted. Again, once I return to the 3DS home screen, the volume goes back to normal.
This seems to happen on every game I tried: Pokemon Trozei!, Pokemon Black, Mario Kart DS, a DSiWare Animal Crossing clock I bought on my 3DS, and I think I tried several other games, too. These things do NOT happen when I play 3DS games on my 3DS, just regular DS games. It also doesn't happen when I play these games on my DS Lite, though I really prefer not to, because of the amount of cartridge-wiggling I have to do for a game to work, not to mention its microphone is dead. If it weren't for the volume thing, this would be a tolerable problem, since Pokemon Black and Mario Kart are still more-or-less playable without the touch screen. But it does kill Trozei! and Scribblenauts. D:
I just got my 3DS for Christmas last year, and it wasn't used or refurbished. It's still under warranty, but I'm really frustrated this is happening within 8 months of buying it.