When you place a dispenser to shoot an arrow at your head it does not collide with your hit box. I a block on the bottom and the a dispenser on top and loaded it with arrows. The I put a lever on the bottom block and stood tight next to the dispenser and flicked it back and forth with the arrow not hitting me. Then I backed away a few blocks and the arrow hit me. Note this only happens when you are right next to the dispenser. In the video I have hit boxes shown so you can see that the arrow goes through my hit box but does not hit me.