If I enter in a lot of clicks rapidly while in Creative, in my inventory, over an item, the item has a chance to be duplicated or deleted, and will keep properties such as name. This allows me to duplicate command blocks or other items.
It does not work in survival.
I made a video illustrating this.
https://www.youtube.com/watch?v=fRptAxtQuiY
It's too big a file to attach.
I tested this in 1.7.9 and 1.7.10, not sure about other versions.