Chinks that are out of place. Looks like a world has been generated with one seed and then the seed has suddenly changed and different chunks are being generated. This is what sometimes happens when playing with a new update on an old world, however this has been a freshly generated world. This has also happened to me in superflat once when random normal world chunks started generating instead of the superflat terrain. This has always happened to me in creative, however that might just be because I generate a lot more creative worlds, especially when testing out new snapshots and such.