A nether portal I have used many times went haywire and sent me to a random location in the nether, with no return portal to the main world.  I died before I could get somewhere safe (of course it put me right next to a fortress with a bunch of wither skeletons).  World was seed #990748179.  When I returned to the game and went back through the same portal, it worked correctly and sent me to the expected location.  It would be nice if Minecraft had a panic button that immediately puts the game in hiatus after a serious game failure, so the user doesn't lose a lot of progress.