This bug arose during the release of the 1.8 Development Snapshots.

I have provided a world download as an attachment which will show you this in action.

When you summon many of the entity "MinecartCommandBlock" stacked on top of each other in previous versions, they would occupy the same Block space when on an activator rail. They now form in a tall tower, and often clip through the rail and the block beneath it if there is no filled space to stop it from clipping all the way through.
This causes the commands in the mine carts to fail to activate in the same (almost instant) order as before, breaking many compact inventions.

Command to replicate: (summon this 7 blocks above an activator rail to witness the full effects)

summon MinecartCommandBlock -14.5 75 20.5 {CustomName:"Reset",Command:"setblock ~ ~ ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~-1 ~ ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~1 ~ ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~-1 ~1 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~1 ~1 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~-1 ~-1 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~1 ~-1 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~-1 ~2 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~1 ~2 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~-1 ~-2 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~1 ~-2 ~-2 minecraft:coal_block",Riding:{id:MinecartCommandBlock,CustomName:"Reset",Command:"setblock ~ ~2 ~-2 minecraft:coal_block",Riding:

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