Getting real sick of your sh*t, Minecraft.
Real fun having a big f*ck off chunk of the World I can't even access or else I just get stuck and die.
I've had this problem as well. In particular, my End has several of these missing chunks (actual missing chunks, not just holes that slowly taper off) that made it very difficult to fight the Ender Dragon. I also experienced it on an old Nether I had (the whole world changed past a certain X coordinate). Does it happen to you in singleplayer? I think I've only experienced it on a local multiplayer server.
Also, one question I have for you guys: how would you recommend going about fighting a Wither that has already been spawned and has killed me and my friend multiple times? We spawned him several thousand blocks away from our main developed area (glad we thought of that) in an open Plains area, and we're having great difficulty killing him. In our initial fight, we almost had him down to half health, but he managed to kill me, and after that he went the opposite way of my friend and started regenerating all of his health. I already tried one of the powerful golden apples; that helped a lot against the wither effect, but he literally flew the other way as soon as I took it. I just don't know how to fight him in the open space. Most of the tips I've read about fighting him involve spawning him in a cave, which doesn't help now. Would Iron Golems help at this stage? Or would he just destroy them all from the air? It also doesn't help that he flies at exactly the same speed as I move on horseback.