Dum  with its whip and pulls Gandalf to the edge of the broken bridge,  the bridge surface is smooth and no ridges are visible all the way  to the edge of the broken bridge. The tracks left by Gandalfs hands  in the dirt on the bridge further indicate that. However, in the  next shot when Gandalf is clinging to the bridge, just before he  says, "Fly, you fools," there is a ridge that Gandalf uses to grab  on that is not present in the previous shot.