Crane was off loading sand blasting skip from the trailer bed. Four wire rope slings were used to lift the skip by attaching onto the loading hitches. One of the slings became caught on the corner of the skip - this was not noticed by the Foreman or Crane Operator. The load swung towards the crane owing to the uneven sling length. To rectify the situation the Crane Operator was instructed to lower the skip back on the trailer bed. Whilst lowering the skip it swung and struck the crane cab window.