In creative, you are able to build 3 blocks down instead of only 2. However, if you stand on a fence/wall (cobble or mossy), you can't.
Comments 3
The fence is not taller than a full size block, nor is the collision box taller than a full size block. You should stand on top of the fence, not slightly higher than it is.
Slabs are a different story.
Fences are 1.5 blocks high (see wiki here).
Unable to reproduce. Note that on a fence and slab your an extra half block up, meaning the 3 blocks down also has to be an extra half up.