Why is my piston powered cobblestone generator crashing minecraft?

I was trying to build an automatic cobblestone generator using pistons, and while I think the machine should work, it instead causes the Universe to implode game to crash.

The stone is created from water and lava mixing next to a "top" piston, this then uses a repeater to transfer power through the stone when it created and activate the piston - pushing it directly downwards. A few levels down is the "output" piston, which uses the same trick detect when a block has been pushed down in front of it, and will then push it sidewards out of the machine.

The top piston has an override "off" switch in it's loop, which works as a start/stop override for the whole machine.

When I switch the machine on Minecraft crashes - I can just hear a piston going and then "Saving Chunks" and darkness...

Luckily, it seemingly crashes in a recoverable way, my world is still playable after a restart, but with a few "phantom" blocks clogging up the machinery...

So, what's going on and how do I fix it?

Oh, and I'm running version 1.9pre3 with no modifications.

Images of the machine:

Top Piston Construction:

Bottom Piston Construction:

"Phantom" Blocks:

This is a bug in the 1.9 pre-release (not sure if just pre-release 3, or all of them).

As you can see here:

It's working as you intended it to in 1.8.1

Turns out I had a wiring error that was causing the bottom piston to be triggered immediately when an incoming block arrived, instead of being delayed by the repeater.

This is because the repeater next to the piston was on top of a block and this block was short circuiting and delivering power directly in to the piston itself, instead of it flowing through the repeater and along the wire around the back of the piston.

I can only assume the machine was trying to push one block in two directions at once (or something similar) and this was causing the game to crash (this, arguably is a bug, the machine should just jam up or something). Not sure about the phantom blocks though, presumably they're an artefact of how the game tracks blocks being pushed.

The solution then, was to remove that block and place the repeater on ground level - correctly delaying the current.