The Bug

The contraption as shown in figure

will create ghost blocks after running for some time.

How to reproduce

1. Load ghostpistons.nbt
2. Activate the observer as marked in figure shown above
   → Ghost blocks will appear after some time as shown in figure

Ghost pistons seem to occur more frequently with some slight lag, thus potentially also more likely on SMP; in order to be able to reproduce this bug better in Creative or on SSP testworlds, insert:
/gamerule randomTickSpeed 100000
the contraption will more likely/faster produce ghostpistons then; if not, play around with the randomTickSpeed, but don't forget to set it back to the default value 3 when you're done.