A man died after collapsing into a water-filled ditch close to a police station shortly after he was released from custody, the police watchdog said.
The Independent Office for Police Conduct (IOPC) found "red flags" were missed at King's Lynn Investigation Centre in Norfolk in July 2018.
Despite being told the man had epilepsy and no access to medicine, the custody sergeant did not seek medical advice.
A misconduct meeting found the officer breached professional standards.
The man had been arrested for being drunk and disorderly at 20:00 BST on 22 July and was released the following morning.
CCTV showed him collapsing into a ditch near a bus stop near to the station. His body was discovered the next day.
The IOPC concluded the custody sergeant "did not appear to have acted on any of the 'red flags' relating to the man's epilepsy and on his police national computer and custody records".
Its report, published on Wednesday, said the officer should have consulted a healthcare practitioner.
The man previously told a liaison and diversion practitioner, who work with vulnerable people in custody, about his condition which was relayed to the officer.
"Relatively simple steps" could have been taken by the officer, including contacting a practitioner or organising police transport, the IOPC found.
In April 2019 the sergeant attended a misconduct meeting, where he was found to have breached the standards of professional behaviour.
He was given management advice relating to appropriate care for releasing detainees in future.
Norfolk Police said it believed it was "an isolated incident" and that an inquest had found no fault with any one officer or the force.
The force said while a further review by a healthcare professional could have been requested, the man could not have been legally detained any longer for that to happen.