Watch the video for explanation!
https://www.youtube.com/watch?v=LNrw5vmOuGE&feature=youtu.be
I guess this is a bad thing, as it wastes a ton of Iron.
Basically, placing a slab on a full block, then dropping Anvils on it, will result in the Anvils not landing, like sand on a torch, but you will only get ONE Anvil, not 4 like seen in the video.
Duplicate of MC-120158. Essentially, what happens is that the second anvil kills the item of the first anvil when falling down, the third one kills the item of the second one, and so forth.