What I expected to happen:
Blocks with missing textures would be of normal block size (consistent to those of its counterparts)

What happened instead:
Blocks with missing texture rendered in hand end up being too large, and their size does not match up to that of a regular block.

Steps to reproduce:
1) Give yourself a block that has no texture in singleplayer and multiplayer.
2) Hold that "missing texture" block.
3) Press F5 and look at what you are holding in your hand.

Minecraft Vanilla v1.8, Default Texture Pack

NOTE: This glitch/bug is also encountered when viewing other players holding a missing texture block in servers.