what I expected to happen was...
have a normal flat singleplayer world to play on.

What actually happened was...:
The world appeared very weirdly. The grass looked like it was swapping around with the other grass blocks. But that's not the worst of it. I saw huge white squares around 10 blocks high seeming to act like a mob. (It bends down like its eating and moves the average animal speed.) Also I placed a lapis lazuli block and it duplicated 8 blocks apart all over the world. And when I broke it they all disappeared. But the biggest game breaker was that npc villages were duplicating all over the world and disappearing. And of course there were massive lag spikes. This is a huge game breaker and I recommend that this gets fixed right away.

Steps to Reproduce:
1. All you really need to do is make a normal flatland with structures, fly around for a minute and you should see it. It worked for me.
2.for the block glitch just place a block

(but to see the block duplicate glitch create another world with no structures because the villages constantly block it.)

p.s here are some pics of the glitches