Moving a ticket between departments has its own method separate from updating a ticket. When attempting to do so the API should warn the developer of the invalid operation by returning a error message that indicates it as such. Currently it returns a 500 error with no description to indicate the issue.

Although I would argue that one should be able to update the departmentId directly through the update ticket method, as that would be the more in line with REST. Otherwise I suggest it should return code 400 with a message like 'Updating the departmentId is not allowed with this operation.' or 'Provided departmentId does not match current state.'

Also I would remove the departmentId from the example in the documentation of the update a ticket method, so as to prevent confusion.