From http://dev.bukkit.org/bukkit-plugins/fun-hard-mode/tickets/187-found-a-bug-infinite-blocks/
reported by Levang, and other duplicates in former issue tracker.

Not doing this the normal way.. sounds stressful, but i found a bug.

With the gravity with Wooden logs, and cobble, also the other things affected by gravity can be abused.
You can do this by: Imagine have 3 wooden logs on top of eachother. Remove the middle one, then chop the wooden log at the bottom, and it will spawn infinitely like this.

-Levang~

Seems related to issues with integrations with block logging plugins from other duplicated tickets.