The bug

The models used for nether portal blocks do not have texture planes on the inside. As a result of this, it's impossible to see nether portal blocks when you're inside of one (MC-180's fix makes this much clearer to see in Survival now).

This is inconsistent with water, which does have texture planes on the inside since water is intentionally designed for the player to be able to be inside of it just like nether portals, and also inconsistent with other blocks such as powder snow which have inner texture planes for the same reason.

When first introduced, nether portal blocks did have textures on the inside. However, these stopped existing in 13w42a, probably as a side effect of MC-34649's fix. 13w42a was released after MC-180 started happening, so this was likely overlooked in the process.

How to reproduce

1. Enter a nether portal
2. Be transported to the other dimension
3. Don't move
4. Try and "see" the nether portal blocks in first person without moving

Expected results

The nether portal blocks would be visible from the inside like they used to be.

Actual results

The nether portal blocks are completely invisible while you're inside them, which is confusing.