Floor glitch
The floor glitch is a glitch that allows Meta Knight and Marth to become stuck inside blocks on a custom stage.
How to perform
On any custom level, make a row of normal blocks. Directly underneath them, put thin panel layers that can be jumped through, so that they are hugging the bottom. Use Drill Rush and aim it into the bottom, and when you are inside the blocks, the character can move inside the block normally. This can also be done by Marth by using Dolphin Slash.
Shuttle loop glitch
To activate the shuttle loop glitch, use the above said stage, but instead of using a Drill Rush into the bottom, jump into it until the player's head is pushing against the bottom, and then use Meta Knight's Shuttle Loop. Doing so should make him go right through the floor, making this a useful sneak attack if the other person doesn't know the glitch.
Alternate method
As a side note, assuming this glitch is supposed to place the player's character inside the row of blocks, there is a much easier glitch. On a custom stage, select the techno/mechanical theme. Next, make a row of the white, cylindrical drum shapes horizontally. Above these, make a row of standard blocks from left to right ending about half way across the drums (or as long as the player wishes, allowing for additions.) At the end of the blocks, place a stair case joining the top of the blocks to the drums (assuming the player went left to right, the top of the staircase should be left, and the bottom, right). Now, if the player gets a run-up from the right of the drums, they should pass through the staircase without climbing up it and be on top of the drums under the blocks. Notes: this glitch was performed on one of Nintendo's "Stage of the day's" using Kirby. Yoshi is also able to perform this using the Egg Roll, but since the CPU achieved this, it is unsure whether he can actually run through it like Kirby can.