Police have been warned to check those they have restrained are not having breathing difficulties.
The Police Ombudsman said a man appeared to "pass out" while being taken into custody.
He had been struggling and aggressive after he was arrested in Derry in September 2018.
His hands had been cuffed behind his back and leg restraints fitted before he was placed into the police car to be taken from Derry to a police custody suite in Coleraine.
The officers involved had requested the use of a cell van, but none had been available.
The ombudsman investigator who dealt with the case, said: "The man continued to struggle but quickly became less active and could be heard telling police that he could not breathe.
"He then began to make gurgling noises, and police body worn video footage shows him becoming quiet and motionless.
"Thankfully police stopped the car and checked on his condition.
"When he came round he almost immediately became aggressive again.
"Officers restrained him, then placed him into a more appropriate upright position between two officers and continued their journey to the custody suite.
"Once there, he was examined by a police doctor."
Ombudsman Marie Anderson said officers should be aware that certain restraint techniques could interfere with people's ability to breathe, with potentially fatal consequences.
She said: "A face-down position is recognised as a risk factor, as are alcohol or drugs.
"It can happen to people who are being restrained, and is also known to be a risk for infants if the positioning of their head and body obstructs their breathing.
"I know that police have provided training on this issue, and guidance for officers is available on the PSNI website.
"I would urge police officers to refresh their knowledge of the guidance so that they remain mindful of the risks of certain restraint techniques and alert to the danger signs of positional asphyxia."
The police officer who had been in the rear of the police car said he could not recall the man having been lying face down for a long period of time.
He acknowledged that he had received training and was aware of police guidance on positional asphyxia, and said he had tried to ensure that the man had enough air and room to breathe.
The ombudsman found that the officer should have been more alert to the risks of a person being restrained while face down, and to danger signs such as the person making gurgling noises.
She recommended that he be disciplined.
The PSNI accepted that there had been failings and the officer was given advice and feedback by his line management.
Superintendent Jon Burrows said: "PSNI acknowledge the findings of the Police Ombudsman, and the officer in question has been given advice and feedback by his line management.
"I want to reassure the public, PSNI officers receive training and guidance on restraint-related risks as part of mandatory training.
"PSNI will further reflect upon the findings of the ombudsman's investigation to identify if there are any other learning opportunities."
