This error can be returned, for example, when attempting to update a shipment_group:

  {
  "reference": "sg_00980781386890476031324735930377",
  "add_shipments": [
    "sp_00980781386816689055029897723911"
  ],
  "remove_shipments": [
    "sp_00980781386816689055029897723911"
  ]
}

In this request, the same shipment reference has been used for both the add_shipments property and the remove_shipments property, which essentially requests that the shipment is both added and removed from the group. This would result in a conflicting_properties response.