When Clarence's jugular vein is severed by Robocop's spike, he falls into the muddy water, blood jetting from his neck. Try to ignore the gore on offer and you will see someone standing against the quarry wall.
How is that a slip-up??? It just a bystander... c'mon. If you were some guy out for a walk and saw a cyborg stabbing a guy in the neck, wouldn't you stop and watch???