There are a lot of questions about Nether portals moving around here, but nothing that I can find that directly answers this conundrum (or I just don't know what to search for). I have a nether portal that just decided to move it's exit, AFTER it had already been used successfully for travel in both ways.
I am playing a normal single player game (so no other players are creating portals in the world), my only mod being a texture pack. I created a nether portal a little while back (just a few weeks, no new releases since then that I know of) and used it to go gather some nether quartz and some other stuff. It all worked fine and I got in to the nether and back several times without problem.
Then today I went in to try and find a fortress. I went quite deep into the nether before I found one, and I proceded to grab all the loot I could carry and then run on home. Except, as you might have guessed by now, this time the portal didn't take me home. It took me to some other unexplored cave in the vicinity of my home (150-ish blocks away from the original portal).
Does anyone know why this might have happened? I have read that the first time you use a portal the exit portal might be forced to move out of a big chunk of solid matter and therefore get pushed "out of range" from your overworld portal, and so spawning a new overworld portal when used. But my portal had already been used successfully to travel both ways, two or three times.
The only difference this time is that I left the immediate area of the portal in the nether which I had not done before. If an exit portal has been pushed out of its intended location, does a chunk have to unload before it "loses track" of the overworld portal?