When placing a dispenser two blocks from the floor (Head height) and shooting arrows (Any type) using it, mobs placed in front of the dispenser within a block of it will not be harmed and instead the arrow passes through them. (example, Zombie/Creeper. Any two block tall+ mob)

I have only tested this on Skeletons (All three types), Zombies (all three types, no baby Zombie), and Creepers. 

I could be missing a lot here, please fill me in if I am.

Update: Fire Charges also phase through enemies if they are one block in distance.