Each year there are reports of spectators sabotaging or booby-trapping the course by digging holes, blocking a river to create a makeshift watersplash, or burying and hiding obstacles. Racers are warned to beware of large crowds of spectators in remote parts of the course since it may indicate hidden traps or obstacle changes. Many of the booby traps are not created to intentionally injure the contestants but are created by the local spectators as jumps or obstacles for their own entertainment and to create intriguing moments to be caught on videotape. The haphazardly designed obstacles, created by the spectators, are often very dangerous as the contestants may inadvertently enter the booby-trap at unsafe speeds, resulting in damage to the vehicles or injuries to competitors or spectators. Awareness of booby traps and course alterations are often part of race-day strategy and convey an advantage to the best prepared teams – nonetheless given the danger the traps pose, it is customary for competitors to quickly communicate course hazards to other competitors through on-board radio communications and radio relay.
From Wikipedia