Floating koopa

From Glitches

From the game Super Mario Bros. for the NES.

You can make a koopa shell float in mid-air one block up from the stage. It is possible in stages 1-3, 3-1, and 3-3. If you hit a turtle flying in the air, and while in the air hit it again while it is 1 block above the bottom of the screen, it will float there. If you wait, it will start walking in mid-air! If you hit the shell twice while in the air, but not 1 block away from the bottom of the screen, the turtle shell will turn upside-down.

Submitted by: andrew gardikis