Tested in creative, having a powered dispenser with a fast-clock update, shoot arrows into stairs/slabs will sent the arrows to the player's inventory up to 10 Blocks away.
Also, while this is running, the button does not get pushed out until the arrows are finished or the clock is interrupted.

What I expected to happen was:
Arrows to get stuck in stairs/slabs

What actually happened was:
Instead, Arrows was sent to my inventory when within 10 blocks of dispenser.
Button return can not update until redstone is broke or arrows finish.

Steps to Reproduce:
1. Place a dispenser
2. Place stair/slab in front of the dispenser
3. Add arrows in the dispenser
4. Connect Redstone to trigger dispenser
5. Move away, and notice arrows flying at you!