On servers with 1.4 maps certain abnormal chunks in the nether replace the normal ones.

On 1.4 servers if you go into the nether there are chunks that have random blocks. Some chunks are the normal ones with nether rack and lava. Most of the chunks get replaced with corrupted chunks which have blocks like end portal blocks, command blocks, pig spawners, and water. Also enderpearls can still be thrown from normal chunk to normal chunk, but if you through an enderpearl at a corrupted chunk you will be floating much as if you fell into an unloaded chunk in the real world.