When I join a world, sometimes large chunks of 16 x 16 terrain appear. The chunks of terrain seem to have switched with another biome near it. For example, I built something near a desert forest and hills. A few log ons later there was a few large squares of forest and hills terrain in the desert at a different elevation. In another instance, a 16 x 16 part of a house I had built previously had been duplicated and generated in a river, keeping the same elevation as the original.They usually appear after about 5 log ons. This only seems to occur on 1.11 and 1.12.