One rig floor winch had been previously used by an individual to lower lift caps down the Pipe Ramp to the Catwalk. Another employee came along and attempted to retrieve this hoist line from the Catwalk. While attempting to retrieve the winch line, the individual mistakenly operated the control on the wrong drill floor winch. The winch he activated was tied off to a TIW valve. When this hoist was activated, the TIW valve was lifted clear of its storage rack and swung across the drill floor, striking and injuring another employee.