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Runners at a 10k trail race in Glasgow were sent in the wrong direction by a prankster pretending to be a marshal and who buried arrow signs marking the course.
Acorn Trails said about 30 of the 115 participants at the event on Sunday were sent on a 2.5km (1.5 mile) detour by the woman.
Organisers Alex Osborne and Michelle Sinsheimer told Sky News it is "hard to laugh off the incident" as the prank led the runners away from medical help had it been needed.
Mr Osborne said: "It's hard to laugh off the incident when you consider the possible implications had things not panned out as lightly as they did.
"There's a fine line between it being a humorous prank and an incident had one of the runners, not all of whom were local, got more lost when their warm clothing, refreshments and first aid were at the finish line.
"I'd ask the saboteur, who we were told is female, to consider the above, which I'd like to think they hadn't thought of on Sunday."
The competitors affected by the "mischievous" interference have been offered a 50% discount on their entry to the next event.
The near 10k race starts and finishes at the old golf clubhouse within Linn Park following two laps of Castlemilk Woodland.
Marshals had been positioned at several turning points, but the woman was able to direct runners towards Linn Park Bridge from an unmanned spot.
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Acorn Trails has held multiple events at the park, with at least three going into Castlemilk Woodland.
Mr Osborne said: "We've never had issues before. Maybe the odd sign tossed into a hedge, but never a course alteration or pretending to be part of the event staff.
"In terms of stewards at future events, we rely on volunteers giving up their own time and can only deploy the people we have available.
"We target spots where we think runners might get lost and where they appreciate the vocal support our amazing hi-vis heroes provide.
"It's not practical to have marshals in the place of every arrow sign as that would require a huge volume of people."
Acorn Trails hosts a number of events throughout the year, with both runners and volunteers invited to get involved.
(c) Sky News 2025: Female prankster pretending to be marshal sends Glasgow runners on 2.5km detour during race