Sometimes when there has been an amount of lag caused by pistons moving to an observer clock, when you break the clock and place a comparator where one observer was it will glitch out and a constantly moving observer block will appear visually.

Nothing should have happened. No visual effect of a ghost observer. 

Steps to Reproduce:

I put pictures to help see how to build the machine.

1. Place out pistons in a line all facing out to the side, you will want to have atleast 29 total (see picture)

2. Place a block behind each piston and put a piece of redstone on it so it creates a line that will power the piston.

3. You may have to see the pictures for this, place one observer one back from the redstone in the exact middle of the redstone line so it will power every piece of redstone.

4. Place a sticky piston 1 block behind the observer and two blocks above, facing downwards.

5. Place an observer under the sticky piston (so it is not touching the other observer yet) with the face of that observer facing the direction of the face of the other observer.

6. put a redstone block on top of the sticky piston so it will push the observer down and form a clock, do not destroy the redstone block or piston.

7.  Destroy the observer directly under the piston and quickly place a comparator. 

If done correctly it should produce the visual effect seen in the pictures.

However this part is tricky because it didn't always work for me on a consistent basis. It seems to be based on the amount of lag the pistons produce.