When in single player and world is stressed, gravity block fail to show their animation while falling.
To reproduce:
Run a world that requires your computer to work hard cpu wise(for me its a custom amplified world.... Effect does not happen on super-flat)
Place gravel/ sand/ anvil in a pillar and break lowest block.
Environment
Java 7 Update 67 (64-bit); Intel Core i5 2.4 gHz; Intel HD Graphics 3000; Windows 7
Fancy graphics; 12-chunk render distance; VBO's ON; Alternate blocks OFF; VSync ON
Fixed for 1.8.1-pre2