Using Buildcraft @ tip of 6.5.x, namely d7d7585 :

I've found several situations where the builder dupes items:

• Chest containing a single pickaxe: requires a pick to be in the Builder's inventory but doesn't consume the pick. Changes the Builder's pick's damage value to match that of the duped pick.
• Chest containing 65 to 127 of one item: Requires and consumes 64 items, but places a chest with the full number of items. 128+ of the item and the chest can't be placed.
• Chest containing chests: The number of chests required is 1 too small (presumably it isn't counting the block chest, and only the item chests?). Ex. A chest with one chest inside requires and consumes only one chest.

I am guessing 2 and 3 are related, 1 independent.