When the leaves of a tree are decaying, a sapling or wood block that is near the edge on top of a leaf block, will sometimes fall down through the leaf block onto the ground, but it cannot be picked up, or at least it is picked up and then immediately appears back on top of the leaf and falls back through again. It can only be fixed up if the leaf block it rests on is destroyed, or if you go on top of the leaf to pick up the item. The item should not fall through unless the leaf block it is resting on is destroyed.