When using custom block models for solid/non-transparent blocks (like stone), the side of the blocks adjacent to the custom model won't show, even if the custom model allows for transparency.

Attached is an example of that.

As you can see, I can see through part of the block into the terrain below, because it is only 14x14x14 voxels and has a hole in the middle.
While this is a nuisance for resourcepack makers, it is also a possible xray exploit.