Not sure if this is a bug or intentional, but if you are sneaking and moving forward, you cannot fall off the edge of a block. However, if you jump while sneaking and moving forward, you can fall off the edge of a block. It seems that while sneaking, you should not be able to move forward past the edge of a block to a different vertical level (as if there is an invisible wall there). Right now this only seems to be a property that exists while you are standing on a solid block.