I created a nether portal at roughly 1000 x, -2000 z. It corresponds to a portal in the nether at roughly 150 x, -250 z. The portal worked before. However, the last time I used the portal, it took me to roughly 1000 x, -2000 z (in the nether). It basically seems to have taken me to the nether without actually changing my coordinates. It did not spawn another nether portal, and indeed it placed me inside of netherrack, fairly deep in.