The bug:

Players can enter the nether portal outside the portal.

How to produce:

1. Build a nether portal like this:
2. Move forward until you touch the obsidian at the bottom of the portal.

If the player is in creative mode, the player will come directly to the lower realms. If the player is in survival mode, the screen will distort as if you were inside a portal.

The player should be within the overworld and the viewpoint should be displayed normally.