Upon going through an overworld nether portal. A player sometimes will be generated into a random location (up to thousands of blocks away). They then have no visible nether portal unless they place blocks randomly at the right spot beside the portal to make it appear. If they go back through the portal they will then be sent even further into the overworld some 5 to sometime 10k blocks. I've seen quite a few people including myself with this bug over the last few months.