Minecraft wiki entry for Cactus_farming, associated text: Items can also hit the block above a cactus. If they do so, they can lose their momentum and fall back down onto the cactus. A half block space above the grown cactus should be sufficient to prevent this.
As noted in the video below, there is one exception to the above: End rods will not break an adjacent cactus block, so they can be used to further guide the dropped items.