"Misdiagnosed in A & E Dept."

On Friday 19th feb. my brother in law was taken to A & E after a fall where he damaged his shoulder. He was discharged saying the x ray results said No breaks sent home with no pain relief even though he was in agony. 2 nurses told him to get off couch but he shouted out he couldn't move his arm and the one nurse stood back and said well you will have to as you are going home. As he lives on his own we took him back to stay with us. Over the weekend we got concerned with the pain he was in and realised this was no frozen shoulder!!. Monday 22nd feb we went to his local hospital Telford royal within an 1 hr the doctor said he needed to be operated on that he had a reverse dislocation and they had to get a surgeon from outside to fix it on Tues 23rd feb.

How can this misdiagnose take place?