The standard orienteering event is a point-to-point race; controls are numbered
on the map and connected in the order the competitor is to visit them. Upon
reaching each control, the orienteer punches a pattern
in the corresponding numbered box on the control card. This allows the event
organizers to verify that the correct controls were visited. Sometimes an orienteer
accidentally punches in the wrong box on the card; if this happens, the correct
procedure is to punch in the correct box, and/or to punch in any of the boxes
on the card that would not normally be used (for example, #20 could be used
if the course has 12 controls) until punching in the correct box can be resumed
(and it never hurts to explain what you did to someone at the finish line).
Note that one must take reasonable care to punch in the correct square - failure
to do so by an experienced orienteer, may result in disqualification.