Description:
One block (both transparent or not) has no backface culling whilst any others
do. This happens for nether portals and "Xray-blocks" (Glowstone, TNT,
Redstoneblock).

Effects:
Nether portal: One block is less transparent than the others
Xray-Blocks: Can't look outside when standing in the block

How to reprocude: (For a nether portal)
1. Stand inside a lit nether portal (you have to do it before it teleports you!)
2. Place a block (e.g. Torch) somewhere else
3. The portal block, your head was in, will turn less transparent and be
visible, when standing in the center of it.
4. Placing or removing any (nearby) block fixes it as well as reloading (F3+A)