Confused runners in the Carnegie Harriers' Devilla Forest 15km trail race near Kincardine, Fife, were faced with arrows pointing towards them instead of away from them.
The mystery sign switching cut about one-and-a-half miles off the route of Sunday's race, in which about 500 runners took part.
Many had posted times that they wrongly believed were personal bests.
Harriers president Malcolm Smith wrote on Facebook: "Those that participated in the 15k race will be aware that there was a problem with the race route. Prior to the race, the route was set up and checked by members of the club and all was in order at that stage.
"However, shortly after the race began, it became apparent there was a problem with the route, with all runners going off course and concerns were made by marshals to the Race Director."
Competitor Mike Smith, 50, said: "It's a case for Rebus. Everyone was very confused."