When i execute replaceitem for slot.weapon.offhand with pumpkin it tells me that slot 99 is replaced, but command testfor for slot 99 doesn't find item in it.
To reproduce:
/replaceitem entity @p slot.weapon.offhand minecraft:pumpkin
output: Replaced slot 99 with 1 * [Pumpkin]
/testfor @p {Inventory:[

]}
output: [nick] did not match required data structure