I got a stack of one of the new arrow types, and put two of them in a dispenser and triggered the dispenser. When I picked up the arrow the dispenser had shot, instead of giving me 1 it gave me two arrows back, filling the stack again and of course I had one more in the dispenser - I now had one more than I started with. I am unsure under what conditions this can be reproduced, or indeed if it can be at all, but probably worth investigating further.