A passenger train travelled some 16 miles for 23 minutes at 80mph with a door open, it has emerged.
An alarming photo shows a gap in the carriage of a Greater Anglia train as it moves eastwards from London Liverpool Street to Southend, Essex.
The blunder, which happened on August 22, has been slammed as "dangerous" and is now subject to an investigation.
One concerned passenger reported the fault to the driver at Hockley station in Essex, by which the service had travelled for more than 20 minutes.
Luckily no one was hurt.
Greater Anglia said the carriage was immediately taken out of service.
But Derek Monnery, chairman of the Essex Rail Users Federation, said it was an unusual incident and it may have been down to recent refurbishment work on Greater Anglia's rolling stock.
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He said: "It could be a dangerous situation if the train is crowded and a lot of people are standing there, which fortunately it doesn't sound like it was in this case.
"It should never have happened."
A witness who was not on the train but has decided to stay anonymous told BBC News : "All of a sudden there was a technician there looking at the door at the side. It must have been loose because they were able to slide it back."
Now the Rail Accident Investigation Branch (RAIB) is exploring what led to the blunder.
Martin Moran, Greater Anglia commercial and customer service director, said: "Safety is our highest priority.
"We immediately took the train out of service when this happened and carried out our own investigation into the incident.
"We have also carried out checks on every single door on that type of train that we have.
"No one was injured in this incident and there have been no further incidents since."
