Mine was in Trace Memory (Another Code) for the DS.
Spoiler:At one point in the game you're supposed to make a stamp mark on a page or something. You can bring up the stamp on one screen and see the page on the other. I could not figure out how to stamp the page, until I thought to try closing the DS to bring the two together. I was afraid this would just put the DS in sleep mode, but it worked!
I couldn't believe it when it worked. The fact that they had made such clever use of the DS was so fantastic at the time.
Spoiler:There was another part in that same game when you had to blow in the microphone to blow dust off of something. In retrospect it seems obvious, but no other game had ever used the functionality of the DS like that.
Good times.
