The blame for the incident was laid firmly at the door of amateur rider Jacqui Coward, who was leading the Glenfarclas Cross Country Handicap Chase aboard Wedger Pardy as the field turned onto the racecourse proper in the home straight but went wider and ended up on the old chase track.
Coward admitted responsibility and was handed a 12-day ban, as was Adrian Heskin on A New Story, who was eventually brought down in a nasty incident.
Richard Johnson was attempting to change tack on Balthazar King but his mount crashed into the rails and fell, while Emmett Mullins (Uncle Junior), Andrew Glassonbury (Double Dizzy), Patrick Mullins (Scotsirish) and Ken Whelan (De Danu) also missed the turn.
The last two retraced their steps to complete.
This left Garde Champetre (6-1) in front to complete his sixth and by far his most unlikely victory over the course.
Trainer Enda Bolger, a master of the discipline, suddenly found himself with a one-two as Chamirey finished runner-up.
Coward’s dates will be notified, while Heskin must sit out December 26-January 6.
“There’s no way I would have won otherwise, maybe I would have finished fourth at best, but I saw them taking the wrong route,” said winning jockey JT McNamara.
“My horse knows the way better than me!”
Bolger added: “Lady Luck was on our side.
“I thought I was going for seventh and eighth place but I ended up getting the first two – that’s cross-country racing.
“My two jockeys walked the course. I’m not saying the others didn’t, but perhaps we had the most experienced jockeys in this race.”
Many top pilots have stopped riding in these races for fear of picking up suspensions – there was a similar incident in 2004 when Czech raider Registana took the wrong route when in front – but the stewards took a fairly lenient approach.
Stipendiary steward Paul Barton said: “There was no doubt Miss Coward took the wrong course but there are varying degrees of doubt about the others.
“Looking at the film, we felt those riders on the outside didn’t have a chance to take the correct route and if there is doubt, then you have to give the riders the benefit.
“Adrian Heskin was suspended as he appeared to be going the correct way but then changes his direction on to the wrong course.”
Coward, who is a granddaughter of trainer Mick Easterby, held her hands up afterwards.
“I took the wrong course,” she said. “Maybe it isn’t flagged that well but it was my fault. I was leading round the home bend, and was looking for the hurdle, but it wasn’t there.
“I just don’t know what was going on – it all happened very quickly.”
Meanwhile, British champion trainer Paul Nicholls registered the 150th Cheltenham winner of his training career as hot favourite Oscargo landed the Citipost Handicap Hurdle.
The Ditcheat handler had three favourites beaten earlier in the day at Prestbury Park but Oscargo made no mistake in the hands of Ruby Walsh.
The 4-6 favourite briefly looked in trouble coming down the hill but flew up the home straight to score by nine lengths.
Nicholls said: “This was the nap of the day.
“It has been one of those weeks with injuries to horses and suchlike – lots of ups and downs – but it’s great to get a winner at Cheltenham.
“I just want to look to win number 151 now.”