Say you enter a Nether portal from the Overworld with coordinates somewhere around X: -700 Z: -500 where normally you'd be taken to your Nether base with the coordinates X: -100 Z: -100. Now it will instead take you to the Overworld coordinates but in the Nether at X: -700 Z: -500. It will not create a new portal, you will just spawn in a wall or immediately fall down to the ground.