When I get stuck in a portal loop due to lag, sometimes the only way to stop is to close out my client. Upon my return i frequently find myself falling to my death in some random place (so i thought). Then I realized that I had falling in the overworld at the location(X, Y, Z) of the portal that was located in the nether. 90% of the time I would appear that the nether portal coords in the overworld and the rest of the time I would appear at the overworld portal coords in the nether falling to a lava death.