Create a new world. Fly around for a bit. Hit ESC and change your render distance up. Load some new chunks... you'll get this strange occurrence. Save and quit will not help. It appears that the world will be like this permanently. The world generation flickers between different options every few ticks. These screenshots were taken within a few seconds of each other. This may affect more versions; I am not sure.
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In the bottom four pictures, there are giant texture bugs in the bottom right corner. I believe that these are the mobs spawning in the glitched chunks.