In a survival server, quickly switching from one item to an Efficiency shovel and destroying some dirt creates a "ghost block" occasionally.

The block should have disappeared and dropped an item, but the block just disappears. No item is dropped, and when you try to walk into it you get pushed back by the server.

In other words, when quickly destroying dirt, the server doesn't become aware of the client's actions. The client thinks the block has been destroyed, but the server is not aware of this.

To reproduce:

On an SMP server, get an Efficiency V diamond shovel and a pickaxe. Go dig somewhere. Quickly switching from the pickaxe to the shovel to break some dirt will occasionally produce this bug.