This is another Dark World in Super Mario 64 DS. To do it, perform Walk like Normal Underwater in the Castle Grounds. Go over to the waterfall. Now, fall into the pit that you come out of after the secret Stage under the moat. You're trapped. You fall down too far to jump out. (although it may be possible for mario can Wall Kick out. Not sure. Anyway, no matter what jumps you use, you will just fall back in. It's definately a Dark World, albeit a minor one.
NOTE: You must have unlocked Wario and entered the secret stage under the moat first! ... I think.
Why it works Edit
Self explanatory. you are walking where you shouldn't be able to walk, and the pit is too deep to jump out of.
This glitch was discovered by Uberninja1337.