Minecraft wiki entry for World_boundary, associated text: A fake swamp biome past X/Z: ±30,000,000. There is only dirt on the seafloor. It also does not generate trees and tall grass.
A mushroom island biome in the world boundary generating without mushrooms or huge mushrooms.
Trees that are generated close to the world boundary stop being generated. Trees may also appear strange, like this.
Village generated on the world boundary of a superflat world.
A view under the fake chunks (1.6.2)
The corner of the world boundary in 1.2.5. Normal terrain is at the bottom of the image; fake chunks are at the top. Note there is neither sand nor clay on the seafloor.
The player's head unnaturally twisting, as seen in Beta 1.8.
The unnaturally twisting head as seen in second person view in 1.0.0.
A frozen player in 1.2's development with the head no longer capable of twisting.