I've experienced an exploit using the block breaker and the block placer :

Place a block breaker and a block placer so they both act on the same block.
Place any block in the block placer (tested cobble and stone)

Use a lever (and some redstone) to activate the two blocks at the same time.
The first iteration works as expected, the stone is placed and mined as cobblestone
The second iteration does the same but here comes the exploit, from here (this iteration included), the block placer will have a diamond pickaxe in its inventory.
You can pick the cobblestone and place it in the block placer again and you win a new diamond pickaxe !

I've been using the Direwolf20 modpack 1.0.3 for MC 1.6.4 which contains OpenBlocks-1.2.2.jar

Known bug, already fixed in dev (#216)