So I Spawned a MinecartSpawner with a commandblock. The command was the following: (edited to the 1.8 format but the same thing happened in 1.7)

/summon MinecartSpawner ~ ~6 ~1 {SpawnRange:3,MinSpawnDelay:3,MaxSpawnDelay:3,SpawnData:{Block:redstone_block,Time:1,DropItem:0,Riding:{id:"FallingSand",Block:lapis_block,Time:1,DropItem:0}},EntityId:"FallingSand",SpawnCount:1}

This creates a MinecartSpawner that spawns 2 FallingSand riding eachother, so what I expected to happen is that the MinecartSpawner would spawn a Lapisblock (id 22) with a redstoneblock (id 152) on top.

However, it often spawns just a lapisblocks and then later is spawns another one on top of it.

From 14w06a on, Some of the spawned blocks are invisible. They become visible again after relogging.

How to reproduce:
1. Put the command in a commandblock.
2. Power the commandblock
3. Notice that the SpawnerMinecart often spawns 2 lapisblocks instead of a lapisblock with a redstoneblock on top.
4. Notice that some of the spawned blocks are invisible.