This Youtube video I made here shows the glitch and how to repeat it is in the description:
https://www.youtube.com/watch?v=7ynw9wb_RdA&feature=youtu.be

The glitch is that after a set of steps in creative mode, an impulse command block will be non-existent in the inventory. Or, a ghost block glitch. It will appear as normal, but /clear won't clear it and it will not be placed down.

To repeat it, pick-blocke a repeating command block, place it down, pick-block an impulse command block, open your inventory, get rid of the repeating command block, then it is now a ghost block.