Dropping 2 or more Anvils on a Slab will result in you only receiving 1 Anvil.

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.