What I expected to happen was...:

when i use the command:

 /replaceitem entity @e[r=7] slot.weapon minecraft:skull 1 2

should the armorstand get a skull in his hand

What actually happened was...

when i use the command:

/replaceitem entity @e[r=7] slot.weapon minecraft:skull 1 2

i get this error:

Could not replace slot 99 with 1 * [Zombie Head]

and the armorstand doesnt get the skull in his hand

Steps to Reproduce:

1. place an armorstand somewhere

1. type this command while you stands close to the armorstand:

   /replaceitem entity @e[r=7] slot.weapon minecraft:skull 1 2

extra info:

you can replaceitem in the weapon slot with other items like a chest...

/replaceitem entity @e[r=7] slot.weapon minecraft:chest 1 0

you cant replaceitem in the weapon slot with skulls on other entities*
*(i have only tested this on: zombies, skeletons and armorstands)

you can summon a armorstand holding a skull in his hand...