In the latest snapshot, the terrain preforms this weird jiggly disjointed dance (it's very hard to explain, just watch the video clip) near the edge of the Minecraft world (around 29,999,000 blocks from 0,0). It's very extreme near the edge, but gets gradually less severe the closer to the center of the world you get. Amazingly, it's still perceptible (although just barely) as close as 50,000 blocks from the center. This was not an issue at all in previous snapshots.
It reminds me of the the slight jigglyness in pre- Beta 1.8 when you got far away from spawn (like in the later episodes of kurtjmac's Far Lands or Bust), but much more intense.
Also note how the entities (animals) and the wireframe block outlines display correctly, unlike the rest of the terrain.
While I understand this is not a gamebreaking bug, I hope it's fixed, as doing anything far away from spawn (for whatever reason, maybe mapmaking, ect.) would be at worst impossible, at best very unenjoyable.

Ps. Sorry for the low quality of the video, I had to reduce the resolution and bitrate a lot to get it under the 10 MB upload limit.

"Each Minecraft world is larger than the surface area of the Earth, but most of that space is completely f***** up" -Ian