A race had to be dramatically halted because the starting stalls could not be removed from the track.
It became clear there was a problem when officials attempted to flag down the field rounding the home turn at Chelmsford on Saturday.
Nine runners and riders were travelling at more than 35mph but had to abruptly pull up to avoid the stalls, which were still on the track at the entrance to the home straight.
All horses and jockeys were unharmed and the race was voided.
Jane Chapple-Hyam, who trained runner Salamanca, told the Racing Post: “When they load horses up in the stalls they turn the tractor off and I believe when the horses jumped out the tractor wouldn’t go into gear so they couldn’t get it off the track.
“They didn’t have enough time to send the flagmen to the back straight. It was quite scary.”
Jockey Alistair Rawlinson, who was aboard Penzance, felt all riders involved deserved credit for safely pulling up and avoiding any accidents.
“We were sat second so were one of the first to arrive on the scene – it could have been a disaster,” he said.
“The lads have shown a lot of horsemanship to stop in a short period of time when we’re just about to hit top gear.”
A racecourse statement said: “The safety of our participants is, and always will be, our top priority.
“We are deeply relieved that no harm came to any of the jockeys, horses or staff involved. This evening’s incident is unprecedented in Chelmsford’s 10 years of racing and although the racecourse was not directly responsible for what occurred, we fully acknowledge our role as the racecourse staging the fixture.
“We are committed to working with the British Horseracing Authority (BHA) and our partners to conduct a thorough investigation and ensure that events of this nature do not happen again.
“Forms of reimbursements will be arranged for affected connections and they will be contacted with further information.”
A stewards’ report read: “An enquiry was held into the stop flag being deployed in the closing stages of this race, as the stalls were unable to be removed from the course.
“Attending the enquiry was the director of racing, the clerk of the course and trainee clerk of the course, both starters, jockeys Billy Loughnane and Harry Davies, the stalls team leader and the tractor driver.
“As all the runners had pulled up and the judge was unable to declare a result, the race was declared void and a report was forwarded to the head office of the BHA.”