I opened my DS and there is now a strange marking on the top screen. It looks almost like a sideways mouth or something. Has this happened to anyone else?
The top screen may be broken. Something like this happened to one of mine. The top screen is all screwy, like half is blank and the other is fine. Have you dropped it?
If it's the top screen, did you ever push something onto it hard like a stylus? I believe that could make such a mark, but only temporarily, as it disappears once released.
I'm just stating that one possibility, and that possibility is void because it would have gone away. You most likely won't be able to fix this problem. Either send it in for repairs, or get a new one. Can you repair a DS screen?