Anvil Glitch. Going from Slab to Anvil, you will glitch down before ending up ontop of the anvil.

When Going from a half slab, to an anvil. It glitches you down a little before going on top of the anvil, but does not do it going anvil to half slab. This goes for the same with full blocks. Going from Anvil to a Full Block will glitch you down but not as much. Here is an example http://www.youtube.com/watch?v=PxeX0aMenxY.
See image for how it is set up.