Item Breakage Avoidance
When falling down one tile gap holes with a shell or Goomba in your hand, there may be obstacles on the side that will destroy it. Moving back and fourth as you fall will help preserve the item.
Button Input Header
While holding your shell or Goomba friend with the X/Y button, fall down the gap and press the D pad left or right to avoid the side obstacles. Be careful not hold down the D pad to the sides as you may inadvertently kill Mario by touching whatever is on the sides.