An employee was working on a scaffold setting up a 30" pipe for pre-heat. The pipe was tack-welded where the set-up was taking place. Workers on the other end of the pipe removed the pipe fitting dogs, and 15 minutes later the tack welds at the set-up area failed. The employee's lanyard was between the pipe and the scaffold deck at the time. When the tack weld failed, this caused the pipe to fall and pinch the employee's left knee against the pipe, cracking the fibula.