When an arrow is shot by a dispenser facing a half slab or stairs, the arrow cannot be picked up as item by the player until the block in front of the dispenser is removed. This is different to the behaviour of all other blocks where the arrow can be picked up.
The following is based on a decompiled version of Minecraft 1.10 using MCP 9.30.
The reason for this is that the method net.minecraft.entity.projectile.EntityArrow.onHit(RayTraceResult) sets the position of the arrow to NaN, NaN, NaN because
- raytraceResultIn.hitVec.xCoord - this.posX = 0 (for y and z as well)
- MathHelper.sqrt_double(0, 0, 0) = 0 (= f2)
- this.motionX / (double)f2 * 0.05000000074505806D; = NaN
This causes the bounding box to be invalid as well. This cannot be seen in the NBT data because the method net.minecraft.world.World.updateEntityWithOptionalForce(Entity, boolean) sets the position to previous if it is either NaN or Infinite, however it does not correct the bounding box. Changing the onHit method which causes this first would probably be the better choice.