Mod edit, description from MC-46780:
I created a Custom mob spawner using the setblock command, and after it spawned some entities, "Ghost" entities started appearing out of nowhere, these entites cannot be killed by the command /kill @e, and cannot be interacted with in any way

Steps to Reproduce:
1. Create a Superflat world
2. Teleport to (0,0)
3. Use the setblock command to make a Custom spawner, the one I used was:

 /setblock 27 58 4 mob_spawner 0 destroy {MaxSpawnDelay:1,MinSpawnDelay:1,SpawnCount:1,MaxNearbyEntities:1,EntityId:Item,SpawnRange:0,RequiredPlayerRange:32000,SpawnData:{Item:{id:264,Count:1},Pos:[27.5,57.2,4.5]}}

4. Make the Spawner spawn some entities
5. Notice how out of nowhere "Ghost" entities start to appear

Original description:
When I was creating this custom map in 1.7.4, I wanted to make the map 1.8 ready, so I moved the save file over and I logged in. And I found it. It is a floating Wither Skeleton and floating Blaze on opposite sides of the map. Skeleton is at: 0, 25, -25 and Blaze is at: 0, 25, 25. They are bumping up and down a pixel or so very fast. All the animations on it are right. When I move around in comparison to a block, the Wither Skeleton moves it's location. I can freely build through the Wither Skeleton. It appears to be a graphical glitch or something. Reloading world didn't help, nor did restarting Minecraft. The mobs corresponding spawner is at 0, 50, 50 on both sides. I made the spawners with just the setblock command. This bug does not appear in 1.7.4 when I move the save file in between the versions. It also appears, on the Blaze, when I move around, the particles follow it like it was moving from side to side... They do not despawn in peaceful. I attacked the world, just go through the nether portal at 0,0 in the overworld to get to the area. I do not have any mods installed and I have no idea why this could be happening. I reloaded the world in a different difficulty and it changed from a Wither to a normal Skeleton. Those two are an entity as shown in the F3 screen.

I also did this in the overworld, and the same thing happens, using a pig spawner, a floating pig is now there...

I created a new world in the snapshot and the same thing happens in a normal superflat world too.