It seems to happen also when mobs fall through trap doors. If the trapdoor is next to a door they can glitch through it as well.
Example.
Happens in 1.6.2, mobs sink half a block after falling in a floor trap but if they are heading toward you they just sink half a block and then magically pop back up and bypass it.
Fixed in upcoming 1.8.
http://minecraft.gamepedia.com/1.8
(world generation, cobblestone added under roads to prevent collapse in caves)