*What I expected to happen was...
I was expecting to be able to copy a stack of arrows and a stack of snow balls in a dispenser by pressing the pick block (scroll) button.

*What actually happened was...
In each of the dispensers I set up I was able to copy the arrows and the snowballs in all of the slots of every dispensers except for everytime I tried to copy from the top left square neither the snow balls or the arrows were copied.

Steps to Reproduce:
1. be in creative mode
2. place a dispenser
3. put a stack of snowballs or arrows in the top left slot of the dispenser
4. hover over the item you placed in the top left slot of the dispenser and press the pick block (scroll) button (on your mouse)
Conclusion. the item should not have been copied but if you try to copy an item in a different slot it will copy.