Place a Sponge Down Twice (One ontop of another) Place an Anvil beside it; watch it fall then place a different block where you placed the anvil originally and it puts an anvil there instead and the anvil fails to hit the ground.

This happens in a world that has the height limits set from -1024 to 1023 (When testing on a fresh world I could not reproduce the issue)

The same thing happens for any Falling Block type when placed next to the Sponge.

NOTE: I could not reproduce this on a SOLO world, perhaps this only effects Servers. Will re-post more information if/when I have time to investigate further.