Runners going off course
Runners turning in the wrong direction is one of the worst things that can happen in a race. It's bad enough in a short race, but several hundred runners at the 30th Annual Rite Aid Cleveland Marathon ran an extra 2.7 miles this past weekend.
You can do your best to prevent this from occurring at your race by riding the course on race morning to ensure there are no unexpected barriers awaiting runners.
Assign a car (ideally, a police car with flashing lights) or a bicycle as a lead vehicle and place another bicycle just ahead of the second pack of runners, since they often lose sight of the lead runner and may go off course. Be sure that occupants of all lead vehicles are familiar with the course and know the correct location and direction of all turns.
Post visible directional signs and station informed course marshals at all turns on the course. Place course maps in all runner packets and post enlarged maps in the registration and packet pickup area.
Some race directors will say it is the runners' responsibility to know the course. Realistically, runners are in their racing "zone" and merely following the people in front of them. Give them correct directions so they don't go off course.