This reproducting BUG is old, it exist in Minecraft 1.3.2 do, and be not patched.
Much Admins need a patch to stop it.
It works with sticky pistons and sand/gravel on the top, and sticky pistons are powered with a fast clock, so the pistons push fast sand/gravel up and the game make a new block ower the pistons, and push old block up.
You can reproducting sand/gravel how much you want, and this must be stopped.
Link to YouTube:
Comments 2
MC-51 to be exact.
Already reported, fixed, and 1.4.3 has new bug. I do recommend searching before making a new report.